feat(layout): re-probe the layout when the workspace's plugin states change

The backend now activates layout providers PER WORKSPACE (the workspace's
per-workspace enablement state picks the ONE active layout plugin), so
toggling a layout plugin in the workspace plugins modal changes which tree
serves the workspace. The modal's toggle now refreshes the layout so the
newly-enabled provider's tree (or the no-provider degrade) lands without a
reload:

- WorkspaceLayoutPort gains refresh(workspaceId) — the modal calls it
  after every state write (an attempted write re-probes either way: the
  probe is the idempotent catch-up; a refused write changed nothing, so it
  re-syncs the same tree).
- App.svelte wires the port to the layout store's new
  onWorkspacePluginsChanged(workspaceId) — a guarded re-probe that only
  fires for the ACTIVE workspace (another workspace's toggle is picked up
  by its own probe on switch, exactly like the other §2.6 triggers).

The per-workspace tree resolution itself is backend-side; the marker
renderer, the probe triggers, and the WS  push filtering are
unchanged.

Verification: typecheck (svelte-check 0 errors) / test (1528) / check
/ build all green.

3 files changed, 63 insertions, 3 deletions

diff --git a/src/features/layout/adapter/abi-port.test.ts b/src/features/layout/adapter/abi-port.test.ts
index d11bf9e..24f9c94 100644
--- a/src/features/layout/adapter/abi-port.test.ts
+++ b/src/features/layout/adapter/abi-port.test.ts
@@ -28,6 +28,7 @@ function fakeStore(
     onTreeChange: vi.fn(),
     resetToDefault: vi.fn(async () => null),
     onExtensionsChanged: vi.fn(),
+    onWorkspacePluginsChanged: vi.fn(),
     ...overrides,
   } as LayoutStore;
 }
diff --git a/src/features/layout/store.svelte.ts b/src/features/layout/store.svelte.ts
index 5fdf100..29e2fa1 100644
--- a/src/features/layout/store.svelte.ts
+++ b/src/features/layout/store.svelte.ts
@@ -7,9 +7,11 @@ import { migrationTree } from "./logic/migration";
  * `LayoutHttp` edge (the plugins-store pattern). Owns the ACTIVE workspace's
  * tree and the provider lifecycle:
  *
- * - **Provider probe triggers** — boot, every workspace switch, and every
- *   `extensions-changed` (a provider enabled/disabled mid-session starts/
- *   stops serving; the probe picks it up without a reload).
+ * - **Provider probe triggers** — boot, every workspace switch, every
+ *   `extensions-changed`, and every change to the ACTIVE workspace's
+ *   per-workspace plugin state (a layout plugin enabled/disabled in the
+ *   workspace plugins modal starts/stops serving; the probe picks it up
+ *   without a reload).
  * - **No-provider mode** — a 404 (or network failure) keeps the tree on its
  *   local default; edits still work but are EPHEMERAL (in-memory only —
  *   D-L4 retired the localStorage write path); logged once, no error UI.
@@ -66,6 +68,15 @@ export interface LayoutStore {
   resetToDefault(workspaceId: string): Promise<string | null>;
   /** Multi-client coherence (§11): re-probe on `extensions-changed`. */
   onExtensionsChanged(): void;
+  /**
+   * The active workspace's PER-WORKSPACE PLUGIN STATE changed (the workspace
+   * plugins modal toggled a layout plugin's enablement — the per-workspace
+   * activation rule): re-probe so the newly-enabled provider's tree (or the
+   * no-provider degrade) lands without a reload. A NON-active workspace's
+   * toggle is ignored — its tree is fetched on its next probe (workspace
+   * switch); there is nothing to re-render here.
+   */
+  onWorkspacePluginsChanged(workspaceId: string): void;
 }
 
 export interface LayoutStoreOptions {
@@ -291,5 +302,15 @@ export function createLayoutStore(opts: LayoutStoreOptions): LayoutStore {
         // migration path. Staying silent keeps the degrade quiet.
       });
     },
+
+    onWorkspacePluginsChanged(workspaceId) {
+      // The per-workspace activation rule: only the ACTIVE workspace's tree
+      // can change what is rendered here — another workspace's toggle is
+      // picked up by its own probe (boot/workspace-switch/§2.6 triggers).
+      if (workspaceId !== activeWorkspaceId) return;
+      void runProbe(activeWorkspaceId, false).catch(() => {
+        // Same quiet-degrade guard as onExtensionsChanged.
+      });
+    },
   };
 }
diff --git a/src/features/layout/store.test.ts b/src/features/layout/store.test.ts
index 73850ed..331af95 100644
--- a/src/features/layout/store.test.ts
+++ b/src/features/layout/store.test.ts
@@ -315,6 +315,44 @@ describe("layout store — edits PUT the whole tree + rollback (§2.6)", () => {
 });
 
 describe("layout store — probe triggers (§2.6)", () => {
+  it("a per-workspace plugin-state change on the ACTIVE workspace re-probes (the plugins-modal toggle)", async () => {
+    let absent = true;
+    const served = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    const http = fakeHttp(() =>
+      absent
+        ? { ok: false, notFound: true, error: "No layout plugin is enabled in this workspace" }
+        : { ok: true, tree: served },
+    );
+    const store = makeStore(http);
+    await store.probe("ws-a");
+    expect(store.providerState).toBe("absent");
+
+    absent = false; // a layout plugin got enabled in THIS workspace
+    store.onWorkspacePluginsChanged("ws-a");
+    await vi.advanceTimersByTimeAsync(0);
+    expect(store.providerState).toBe("present");
+    expect(store.tree).toEqual(served);
+  });
+
+  it("a per-workspace plugin-state change on ANOTHER workspace is ignored (its probe fetches on switch)", async () => {
+    let absent = true;
+    const served = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    const http = fakeHttp(() =>
+      absent
+        ? { ok: false, notFound: true, error: "No layout plugin is enabled in this workspace" }
+        : { ok: true, tree: served },
+    );
+    const store = makeStore(http);
+    await store.probe("ws-a");
+    expect(store.providerState).toBe("absent");
+
+    absent = false; // a DIFFERENT workspace's toggle — nothing re-renders here
+    store.onWorkspacePluginsChanged("ws-other");
+    await vi.advanceTimersByTimeAsync(0);
+    expect(http.getCalls).toEqual(["ws-a"]); // no second probe
+    expect(store.providerState).toBe("absent");
+  });
+
   it("extensions-changed re-probes the active workspace", async () => {
     let absent = true;
     const served = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);