7 files changed, 360 insertions, 5 deletions
diff --git a/src/features/layout/adapter/abi-port.test.ts b/src/features/layout/adapter/abi-port.test.ts
new file mode 100644
index 0000000..d11bf9e
--- /dev/null
+++ b/src/features/layout/adapter/abi-port.test.ts
@@ -0,0 +1,96 @@
+import { DEFAULT_LAYOUT, type LayoutTree } from "@dispatch/layout-contract";
+import { describe, expect, it, vi } from "vitest";
+import type { LayoutProviderState, LayoutStore } from "../store.svelte";
+import { abiTree, createLayoutAbiPort } from "./abi-port";
+
+/**
+ * The ABI `layout` port tests (plan 04 §4.5) — the port is pure composition
+ * over the layout store + the ops HTTP edge, so BOTH deps are fakes (the
+ * outermost edges; nothing internal is mocked — asymmetric testing).
+ */
+
+function fakeStore(
+  overrides: Partial<Pick<LayoutStore, "tree" | "providerState" | "activeWorkspaceId">> = {},
+): LayoutStore {
+  return {
+    get tree(): LayoutTree {
+      return DEFAULT_LAYOUT;
+    },
+    get providerState(): LayoutProviderState {
+      return "present";
+    },
+    get activeWorkspaceId(): string {
+      return "ws-a";
+    },
+    probe: vi.fn(async () => {}),
+    onWorkspaceChanged: vi.fn(),
+    onServerTree: vi.fn(),
+    onTreeChange: vi.fn(),
+    resetToDefault: vi.fn(async () => null),
+    onExtensionsChanged: vi.fn(),
+    ...overrides,
+  } as LayoutStore;
+}
+
+describe("abiTree — the getTree decision (pure)", () => {
+  it("a PRESENT provider exposes the tree", () => {
+    expect(abiTree("present", DEFAULT_LAYOUT)).toBe(DEFAULT_LAYOUT);
+  });
+
+  it("no-provider mode (absent) exposes NULL — the local fallback is not shared state", () => {
+    expect(abiTree("absent", DEFAULT_LAYOUT)).toBeNull();
+  });
+
+  it("an unknown (not-yet-probed) provider exposes NULL too", () => {
+    expect(abiTree("unknown", DEFAULT_LAYOUT)).toBeNull();
+  });
+});
+
+describe("createLayoutAbiPort — applyOps → the ops route", () => {
+  it("posts to the ACTIVE workspace and feeds the returned tree back through the store", async () => {
+    const ops = [{ type: "ensurePanel", panels: "side", view: "demo" }] as const;
+    const edited: LayoutTree = { ...DEFAULT_LAYOUT };
+    const store = fakeStore();
+    const seen: string[] = [];
+    const port = createLayoutAbiPort({
+      store,
+      postOps: async (workspaceId, o) => {
+        seen.push(workspaceId);
+        return { ok: true, tree: edited, applied: o.length };
+      },
+    });
+
+    const result = await port.applyOps(ops);
+    expect(seen).toEqual(["ws-a"]); // the ACTIVE workspace at CALL time
+    expect(result).toEqual({ ok: true, tree: edited, applied: 1 });
+    // The successful apply echoes the tree into the store (instant local
+    // re-render; the WS broadcast echo is idempotent with this).
+    expect(store.onServerTree).toHaveBeenCalledWith("ws-a", edited);
+  });
+
+  it("a failed batch (typed no-provider error) does NOT touch the store", async () => {
+    const store = fakeStore({ providerState: "absent" });
+    const port = createLayoutAbiPort({
+      store,
+      postOps: async () => ({ ok: false, error: "no layout provider active" }),
+    });
+
+    const result = await port.applyOps([{ type: "removeNode", target: "side" }]);
+    expect(result).toEqual({ ok: false, error: "no layout provider active" });
+    expect(store.onServerTree).not.toHaveBeenCalled();
+  });
+
+  it("getTree returns the active tree while the provider is present, null when absent", () => {
+    const port = createLayoutAbiPort({
+      store: fakeStore({ providerState: "present" }),
+      postOps: async () => ({ ok: false, error: "unused" }),
+    });
+    expect(port.getTree()).toBe(DEFAULT_LAYOUT);
+
+    const absent = createLayoutAbiPort({
+      store: fakeStore({ providerState: "absent" }),
+      postOps: async () => ({ ok: false, error: "no layout provider active" }),
+    });
+    expect(absent.getTree()).toBeNull();
+  });
+});
diff --git a/src/features/layout/adapter/abi-port.ts b/src/features/layout/adapter/abi-port.ts
new file mode 100644
index 0000000..69576c6
--- /dev/null
+++ b/src/features/layout/adapter/abi-port.ts
@@ -0,0 +1,60 @@
+import type { ApplyOpsResult, LayoutOp, LayoutTree } from "@dispatch/layout-contract";
+import type { LayoutProviderState, LayoutStore } from "../store.svelte";
+
+/**
+ * The port the frontend-plugin ABI exposes as `api.layout` (plan 04 §4.5).
+ * The composition root builds it with {@link createLayoutAbiPort} and the
+ * plugin host hands it to every `activate(api)` — plugins never see the
+ * store or the HTTP edge, only THIS.
+ */
+export interface LayoutAbiPort {
+  /**
+   * Apply op-vocabulary edits to the ACTIVE workspace's tree through the
+   * provider (`POST /workspaces/:id/layout/ops`). Atomic; the no-provider
+   * 404 maps to the typed `{ ok: false, error: "no layout provider active" }`.
+   */
+  readonly applyOps: (ops: readonly LayoutOp[]) => Promise<ApplyOpsResult>;
+  /** The ACTIVE workspace's tree; `null` in no-provider mode. */
+  readonly getTree: () => LayoutTree | null;
+}
+
+export interface LayoutAbiPortDeps {
+  /** The layout store (the active workspace's tree + provider state). */
+  readonly store: LayoutStore;
+  /** The ops HTTP edge — bound per CALL to the then-active workspace. */
+  readonly postOps: (workspaceId: string, ops: readonly LayoutOp[]) => Promise<ApplyOpsResult>;
+}
+
+/**
+ * Build the ABI's `layout` port over the layout store + the ops HTTP edge
+ * (pure composition — no ambient state; both deps injected). A successful
+ * apply feeds the returned tree back through `onServerTree` so the local
+ * client re-renders immediately (the server's WS broadcast is the
+ * authoritative echo for every client, this one included — applying it twice
+ * is idempotent). The workspace is read at CALL time, so a workspace switch
+ * between calls targets the right tree; a switch mid-flight is guarded by
+ * `onServerTree`'s own workspace filter.
+ */
+export function createLayoutAbiPort(deps: LayoutAbiPortDeps): LayoutAbiPort {
+  return {
+    async applyOps(ops): Promise<ApplyOpsResult> {
+      const workspaceId = deps.store.activeWorkspaceId;
+      const result = await deps.postOps(workspaceId, ops);
+      if (result.ok) {
+        deps.store.onServerTree(workspaceId, result.tree);
+      }
+      return result;
+    },
+    getTree(): LayoutTree | null {
+      // `present` = an active provider is serving the active workspace. In
+      // no-provider (and not-yet-probed) mode the local tree is a fallback,
+      // not shared state — null tells the plugin there is nothing to read.
+      return abiTree(deps.store.providerState, deps.store.tree);
+    },
+  };
+}
+
+/** The getTree decision, pure: only a PRESENT provider's tree is exposed. */
+export function abiTree(state: LayoutProviderState, tree: LayoutTree): LayoutTree | null {
+  return state === "present" ? tree : null;
+}
diff --git a/src/features/layout/adapter/http.test.ts b/src/features/layout/adapter/http.test.ts
index 1801ca6..cc7d7e9 100644
--- a/src/features/layout/adapter/http.test.ts
+++ b/src/features/layout/adapter/http.test.ts
@@ -135,3 +135,48 @@ describe("createLayoutHttp — deleteTree", () => {
     expect(result).toMatchObject({ ok: false, notFound: true });
   });
 });
+
+describe("createLayoutHttp — postOps (plan 04 §4.2)", () => {
+  it("POSTs { ops } to the ops route and unwraps { tree, applied }", async () => {
+    const capture: { url?: string; init?: RequestInit } = {};
+    const ops = [{ type: "ensurePanel", panels: "side", view: "demo" }] as const;
+    const http = createLayoutHttp(
+      "http://backend.test",
+      fetchReturning(200, { tree: DEFAULT_LAYOUT, applied: 1 }, capture),
+    );
+    const result = await http.postOps("ws-a", ops);
+    expect(result).toEqual({ ok: true, tree: DEFAULT_LAYOUT, applied: 1 });
+    expect(capture.url).toBe(`${LAYOUT_URL}/ops`);
+    expect(capture.init?.method).toBe("POST");
+    expect(capture.init?.body).toBe(JSON.stringify({ ops }));
+  });
+
+  it("maps a 404 to the typed no-provider error (the ABI's documented failure)", async () => {
+    const http = createLayoutHttp("http://backend.test", fetchReturning(404, {}));
+    const result = await http.postOps("ws-a", []);
+    expect(result).toEqual({ ok: false, error: "no layout provider active" });
+  });
+
+  it("maps a 400 { error, failedOp } to the atomic-reject failure", async () => {
+    const http = createLayoutHttp(
+      "http://backend.test",
+      fetchReturning(400, { error: "op 1: no such node", failedOp: 1 }),
+    );
+    const result = await http.postOps("ws-a", [{ type: "removeNode", target: "x" }]);
+    expect(result).toEqual({ ok: false, error: "op 1: no such node", failedOp: 1 });
+  });
+
+  it("a malformed 200 (no tree in the body) is a failure, never a silent ok", async () => {
+    const http = createLayoutHttp("http://backend.test", fetchReturning(200, { applied: 2 }));
+    const result = await http.postOps("ws-a", []);
+    expect(result).toMatchObject({ ok: false });
+  });
+
+  it("a network failure degrades to a typed failure (never throws)", async () => {
+    const http = createLayoutHttp("http://backend.test", async () => {
+      throw new TypeError("fetch failed");
+    });
+    const result = await http.postOps("ws-a", []);
+    expect(result).toEqual({ ok: false, error: "fetch failed" });
+  });
+});
diff --git a/src/features/layout/adapter/http.ts b/src/features/layout/adapter/http.ts
index 2f91e43..ff15a70 100644
--- a/src/features/layout/adapter/http.ts
+++ b/src/features/layout/adapter/http.ts
@@ -1,11 +1,15 @@
-import type { LayoutTree } from "@dispatch/layout-contract";
+import type { ApplyOpsResult, LayoutOp, LayoutTree } from "@dispatch/layout-contract";
 
 /**
  * Layout HTTP effects (plan 02 §2.6) — the injected edge that talks to the
  * ACTIVE layout provider's well-known routes:
  * - `GET /workspaces/:id/layout` → `{ tree }` (read seeds a fresh workspace)
  * - `PUT /workspaces/:id/layout` body `{ tree }` → 200 `{ tree }` / 400 `{ error }`
- * - `DELETE /workspaces/:id/layout` → the fresh seed (reset; plan 04 adds the UI)
+ * - `DELETE /workspaces/:id/layout` → the fresh seed (reset; the plugins
+ *   modal's "Reset layout to default" button — plan 04 §4.4)
+ * - `POST /workspaces/:id/layout/ops` body `{ ops }` → 200 `{ tree, applied }`
+ *   / 400 `{ error, failedOp? }` / 404 no provider (plan 04 §4.2 — the FE
+ *   ABI's `layout.applyOps` door)
  *
  * Mirrors the plugins adapter's pattern: `httpBase` + an injected `fetchImpl`
  * (testable without the network). The 404 is a SIGNAL, not an error: it means
@@ -22,6 +26,13 @@ export interface LayoutHttp {
   getTree(workspaceId: string): Promise<LayoutHttpResult>;
   putTree(workspaceId: string, tree: LayoutTree): Promise<LayoutHttpResult>;
   deleteTree(workspaceId: string): Promise<LayoutHttpResult>;
+  /**
+   * The plan-04 ops door (§4.2): apply op-vocabulary edits through the
+   * provider. The result IS the contract's `ApplyOpsResult` — the 404
+   * (no provider) maps to the typed `{ ok: false, error: "no layout
+   * provider active" }` failure the ABI documents.
+   */
+  postOps(workspaceId: string, ops: readonly LayoutOp[]): Promise<ApplyOpsResult>;
 }
 
 /** Read an error message from a non-2xx body (`{ error }`), tolerating junk. */
@@ -113,5 +124,52 @@ export function createLayoutHttp(httpBase: string, fetchImpl: typeof fetch): Lay
         };
       }
     },
+
+    async postOps(workspaceId, ops): Promise<ApplyOpsResult> {
+      try {
+        const res = await fetchImpl(
+          `${httpBase}/workspaces/${encodeURIComponent(workspaceId)}/layout/ops`,
+          {
+            method: "POST",
+            headers: { "Content-Type": "application/json" },
+            body: JSON.stringify({ ops }),
+          },
+        );
+        // The no-provider 404 is the ABI's typed signal (plan 04 §4.5).
+        if (res.status === 404) {
+          return { ok: false, error: "no layout provider active" };
+        }
+        if (!res.ok) return await opsError(res);
+        const body = (await res.json()) as { tree?: unknown; applied?: unknown };
+        if (body === null || typeof body !== "object" || body.tree === undefined)
+          return { ok: false, error: "Malformed layout response" };
+        return {
+          ok: true,
+          tree: body.tree as LayoutTree,
+          applied: typeof body.applied === "number" ? body.applied : ops.length,
+        };
+      } catch (err) {
+        return {
+          ok: false,
+          error: err instanceof Error ? err.message : "Layout request failed",
+        };
+      }
+    },
   };
 }
+
+/** Read an ops-failure body (`{ error, failedOp? }`), tolerating junk. */
+async function opsError(res: Response): Promise<ApplyOpsResult> {
+  try {
+    const body = (await res.json()) as { error?: unknown; failedOp?: unknown };
+    const error =
+      typeof body.error === "string" && body.error.length > 0 ? body.error : `HTTP ${res.status}`;
+    return {
+      ok: false,
+      error,
+      ...(typeof body.failedOp === "number" ? { failedOp: body.failedOp } : {}),
+    };
+  } catch {
+    return { ok: false, error: `HTTP ${res.status}` };
+  }
+}
diff --git a/src/features/layout/index.ts b/src/features/layout/index.ts
index 6a17589..8a464fe 100644
--- a/src/features/layout/index.ts
+++ b/src/features/layout/index.ts
@@ -7,6 +7,12 @@
  * OUTSIDE the tree — the tree owns the content region.
  */
 
+export {
+  abiTree,
+  createLayoutAbiPort,
+  type LayoutAbiPort,
+  type LayoutAbiPortDeps,
+} from "./adapter/abi-port";
 export { createLayoutHttp, type LayoutHttp, type LayoutHttpResult } from "./adapter/http";
 export {
   bootDefaultOpen,
diff --git a/src/features/layout/store.svelte.ts b/src/features/layout/store.svelte.ts
index 3d7de3f..5fdf100 100644
--- a/src/features/layout/store.svelte.ts
+++ b/src/features/layout/store.svelte.ts
@@ -54,6 +54,16 @@ export interface LayoutStore {
   onServerTree(workspaceId: string, tree: LayoutTree): void;
   /** A user edit (the panels' dropdowns / TileRoot ops). Persists via PUT. */
   onTreeChange(next: LayoutTree): void;
+  /**
+   * Reset the workspace's layout to the default seed (plan 04 §4.4 — the
+   * plugins modal's "Reset layout to default" affordance:
+   * `DELETE /workspaces/:id/layout` → re-seed → broadcast). Resolves to
+   * `null` on success (the fresh seed is applied locally + pushed via WS to
+   * every client) or to the FAILURE REASON — the no-provider 404 resolves
+   * to the user-facing "no layout plugin enabled" (the button's gate renders
+   * the same reason before this runs).
+   */
+  resetToDefault(workspaceId: string): Promise<string | null>;
   /** Multi-client coherence (§11): re-probe on `extensions-changed`. */
   onExtensionsChanged(): void;
 }
@@ -260,6 +270,21 @@ export function createLayoutStore(opts: LayoutStoreOptions): LayoutStore {
       }, debounceMs);
     },
 
+    async resetToDefault(workspaceId): Promise<string | null> {
+      const result = await http.deleteTree(workspaceId);
+      if (result.ok) {
+        // The server re-seeded and broadcast (every client resets); this
+        // client applies the fresh seed immediately too (idempotent with the
+        // WS echo). Only the ACTIVE workspace's tree is ours to update.
+        notePresent();
+        if (workspaceId === activeWorkspaceId) applyServerTree(result.tree);
+        return null;
+      }
+      // The no-provider 404: the user-facing reason the plan specifies.
+      if (result.notFound) return "no layout plugin enabled";
+      return result.error;
+    },
+
     onExtensionsChanged() {
       void runProbe(activeWorkspaceId, false).catch(() => {
         // runProbe never throws for HTTP failures; this guard is for the
diff --git a/src/features/layout/store.test.ts b/src/features/layout/store.test.ts
index 1d4cdf0..73850ed 100644
--- a/src/features/layout/store.test.ts
+++ b/src/features/layout/store.test.ts
@@ -1,4 +1,9 @@
-import { DEFAULT_LAYOUT, type LayoutTree } from "@dispatch/layout-contract";
+import {
+  type ApplyOpsResult,
+  DEFAULT_LAYOUT,
+  type LayoutOp,
+  type LayoutTree,
+} from "@dispatch/layout-contract";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import type { LayoutHttp, LayoutHttpResult } from "./adapter/http";
 import { overlaySidePanels } from "./logic/seed";
@@ -15,8 +20,11 @@ import { createLayoutStore } from "./store.svelte";
 interface HttpFake extends LayoutHttp {
   getCalls: string[];
   putCalls: Array<{ workspaceId: string; tree: LayoutTree }>;
+  deleteCalls: string[];
   getResult: (workspaceId: string) => LayoutHttpResult;
   putResult: (workspaceId: string, tree: LayoutTree) => LayoutHttpResult;
+  deleteResult: (workspaceId: string) => LayoutHttpResult;
+  opsResult: (workspaceId: string, ops: readonly LayoutOp[]) => ApplyOpsResult;
 }
 
 function fakeHttp(
@@ -32,11 +40,18 @@ function fakeHttp(
 ): HttpFake {
   const getCalls: string[] = [];
   const putCalls: Array<{ workspaceId: string; tree: LayoutTree }> = [];
+  const deleteCalls: string[] = [];
   const fake: HttpFake = {
     getCalls,
     putCalls,
+    deleteCalls,
     getResult,
     putResult,
+    // Re-scriptable outcome (default: the fresh seed a reset returns).
+    deleteResult: () => ({ ok: true, tree: DEFAULT_LAYOUT }),
+    // Re-scriptable outcome (default: the typed no-provider failure the ABI
+    // documents; tests re-assign mid-test like putResult).
+    opsResult: () => ({ ok: false, error: "no layout provider active" }),
     // Delegate to the CURRENT properties so a test can re-script outcomes
     // mid-test (http.putResult = …).
     async getTree(workspaceId) {
@@ -47,8 +62,12 @@ function fakeHttp(
       putCalls.push({ workspaceId, tree });
       return fake.putResult(workspaceId, tree);
     },
-    async deleteTree() {
-      return { ok: false, notFound: true, error: "unused" };
+    async deleteTree(workspaceId) {
+      deleteCalls.push(workspaceId);
+      return fake.deleteResult(workspaceId);
+    },
+    async postOps(workspaceId, ops) {
+      return fake.opsResult(workspaceId, ops);
     },
   };
   return fake;
@@ -424,3 +443,49 @@ describe("layout store — one-way localStorage migration (§2.7)", () => {
     ]);
   });
 });
+
+describe("layout store — resetToDefault (plan 04 §4.4, the reset affordance)", () => {
+  it("DELETEs the workspace's layout, applies the fresh seed, and resolves null", async () => {
+    const custom = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    const http = fakeHttp();
+    // The DELETE returns the FRESH SEED (the server re-seeded).
+    http.deleteResult = () => ({ ok: true, tree: DEFAULT_LAYOUT });
+    const store = makeStore(http);
+
+    // Get to a NON-default tree first: a present provider + a pushed edit.
+    http.getResult = () => ({ ok: true, tree: custom });
+    await store.probe("ws-a");
+    expect(store.tree).toEqual(custom);
+
+    const failure = await store.resetToDefault("ws-a");
+    expect(failure).toBeNull();
+    expect(http.deleteCalls).toEqual(["ws-a"]);
+    // The fresh seed is applied locally (the WS broadcast echo is idempotent).
+    expect(store.tree).toEqual(DEFAULT_LAYOUT);
+    expect(store.providerState).toBe("present");
+  });
+
+  it("a 404 (no provider) resolves to the user-facing 'no layout plugin enabled' reason", async () => {
+    const http = fakeHttp();
+    http.deleteResult = () => ({
+      ok: false,
+      notFound: true,
+      error: "No layout provider is active",
+    });
+    const store = makeStore(http);
+
+    const failure = await store.resetToDefault("ws-a");
+    expect(failure).toBe("no layout plugin enabled");
+    expect(store.tree).toEqual(DEFAULT_LAYOUT); // untouched local default
+  });
+
+  it("another failure (5xx on a present provider) resolves to its error message", async () => {
+    const http = fakeHttp(() => ({ ok: true, tree: DEFAULT_LAYOUT }));
+    http.deleteResult = () => ({ ok: false, notFound: false, error: "storage write failed" });
+    const store = makeStore(http);
+    await store.probe("ws-a");
+
+    const failure = await store.resetToDefault("ws-a");
+    expect(failure).toBe("storage write failed");
+  });
+});