Slice 1: surface system + WS transport + composition root

Pure-core feature libraries assembled at the composition root:
- core/protocol: pure reducer over surface catalog/spec/error messages
- features/surface-host: generic field-kind interpreter (toggle/progress/
  selector/stat/button) + pure plan logic; no surface-id special-casing
- adapters/ws: injected WebSocket client (effects at the edge)
- app: composition root store (Svelte 5 runes over the pure reducer),
  host-relative surface WS URL resolution (resolveWsUrl), root App.svelte

Verified green: svelte-check 0/0, vitest 84 passed, biome clean, vite build ok.

4 files changed, 618 insertions, 0 deletions

diff --git a/src/adapters/ws/index.test.ts b/src/adapters/ws/index.test.ts
new file mode 100644
index 0000000..92b8753
--- /dev/null
+++ b/src/adapters/ws/index.test.ts
@@ -0,0 +1,234 @@
+import { describe, expect, it, vi } from "vitest";
+import type { WebSocketLike } from "./index";
+import { createSurfaceSocket } from "./index";
+
+interface FakeSocket extends WebSocketLike {
+	sent: string[];
+	resolveOpen(): void;
+	invokeMessage(data: string): void;
+	invokeClose(): void;
+}
+
+function fakeSocket(): FakeSocket {
+	let onopen: (() => void) | null = null;
+	let onmessage: ((ev: { data: string }) => void) | null = null;
+	let onclose: ((ev: { code: number; reason: string }) => void) | null = null;
+	const sent: string[] = [];
+
+	const ws: FakeSocket = {
+		send(data: string) {
+			sent.push(data);
+		},
+		close() {},
+		get onopen() {
+			return onopen;
+		},
+		set onopen(fn) {
+			onopen = fn;
+		},
+		get onmessage() {
+			return onmessage;
+		},
+		set onmessage(fn) {
+			onmessage = fn;
+		},
+		get onclose() {
+			return onclose;
+		},
+		set onclose(fn) {
+			onclose = fn;
+		},
+		resolveOpen() {
+			onopen?.();
+		},
+		invokeMessage(data: string) {
+			onmessage?.({ data });
+		},
+		invokeClose() {
+			onclose?.({ code: 1000, reason: "" });
+		},
+		sent,
+	};
+	return ws;
+}
+
+describe("createSurfaceSocket", () => {
+	it("sends queued messages once socket opens", () => {
+		const ws = fakeSocket();
+		const onMessage = vi.fn();
+		const handle = createSurfaceSocket({
+			url: "ws://test",
+			onMessage,
+			socketFactory: () => ws,
+		});
+
+		handle.send({ type: "subscribe", surfaceId: "s1" });
+		handle.send({ type: "subscribe", surfaceId: "s2" });
+		expect(ws.sent).toHaveLength(0);
+
+		ws.resolveOpen();
+		expect(ws.sent).toHaveLength(2);
+		expect(JSON.parse(ws.sent[0] ?? "")).toEqual({ type: "subscribe", surfaceId: "s1" });
+		expect(JSON.parse(ws.sent[1] ?? "")).toEqual({ type: "subscribe", surfaceId: "s2" });
+	});
+
+	it("sends immediately when socket is already open", () => {
+		const ws = fakeSocket();
+		const handle = createSurfaceSocket({
+			url: "ws://test",
+			onMessage: vi.fn(),
+			socketFactory: () => ws,
+		});
+
+		ws.resolveOpen();
+		ws.sent.length = 0;
+
+		handle.send({ type: "subscribe", surfaceId: "s1" });
+		expect(ws.sent).toHaveLength(1);
+	});
+
+	it("routes inbound messages to onMessage via parseServerMessage", () => {
+		const ws = fakeSocket();
+		const onMessage = vi.fn();
+		createSurfaceSocket({
+			url: "ws://test",
+			onMessage,
+			socketFactory: () => ws,
+		});
+
+		ws.resolveOpen();
+		ws.invokeMessage(JSON.stringify({ type: "catalog", catalog: [] }));
+		expect(onMessage).toHaveBeenCalledOnce();
+		expect(onMessage).toHaveBeenCalledWith({ type: "catalog", catalog: [] });
+	});
+
+	it("drops malformed inbound messages silently", () => {
+		const ws = fakeSocket();
+		const onMessage = vi.fn();
+		createSurfaceSocket({
+			url: "ws://test",
+			onMessage,
+			socketFactory: () => ws,
+		});
+
+		ws.resolveOpen();
+		ws.invokeMessage("not json");
+		expect(onMessage).not.toHaveBeenCalled();
+	});
+
+	it("auto-reconnects on close and fires onReopen after successful reconnect", () => {
+		vi.useFakeTimers();
+		try {
+			const sockets: ReturnType<typeof fakeSocket>[] = [];
+			const onMessage = vi.fn();
+			const onReopen = vi.fn();
+			createSurfaceSocket({
+				url: "ws://test",
+				onMessage,
+				onReopen,
+				socketFactory: () => {
+					const ws = fakeSocket();
+					sockets.push(ws);
+					return ws;
+				},
+			});
+
+			expect(sockets).toHaveLength(1);
+			sockets[0]?.resolveOpen();
+
+			// Simulate close
+			sockets[0]?.invokeClose();
+
+			// Fast-forward past the backoff delay
+			vi.advanceTimersByTime(600);
+
+			expect(sockets).toHaveLength(2);
+			// onReopen should NOT have fired yet (socket not open)
+			expect(onReopen).not.toHaveBeenCalled();
+
+			sockets[1]?.resolveOpen();
+			expect(onReopen).toHaveBeenCalledOnce();
+		} finally {
+			vi.useRealTimers();
+		}
+	});
+
+	it("does not fire onReopen on initial connect", () => {
+		const ws = fakeSocket();
+		const onReopen = vi.fn();
+		createSurfaceSocket({
+			url: "ws://test",
+			onMessage: vi.fn(),
+			onReopen,
+			socketFactory: () => ws,
+		});
+
+		ws.resolveOpen();
+		expect(onReopen).not.toHaveBeenCalled();
+	});
+
+	it("close() prevents further reconnects", () => {
+		vi.useFakeTimers();
+		try {
+			const sockets: ReturnType<typeof fakeSocket>[] = [];
+			const handle = createSurfaceSocket({
+				url: "ws://test",
+				onMessage: vi.fn(),
+				socketFactory: () => {
+					const ws = fakeSocket();
+					sockets.push(ws);
+					return ws;
+				},
+			});
+
+			sockets[0]?.resolveOpen();
+			sockets[0]?.invokeClose();
+			handle.close();
+
+			vi.advanceTimersByTime(10_000);
+			expect(sockets).toHaveLength(1);
+		} finally {
+			vi.useRealTimers();
+		}
+	});
+
+	it("close() prevents further sends", () => {
+		const ws = fakeSocket();
+		const handle = createSurfaceSocket({
+			url: "ws://test",
+			onMessage: vi.fn(),
+			socketFactory: () => ws,
+		});
+
+		ws.resolveOpen();
+		ws.sent.length = 0;
+		handle.close();
+
+		handle.send({ type: "subscribe", surfaceId: "s1" });
+		expect(ws.sent).toHaveLength(0);
+	});
+
+	it("queues multiple sends before open and flushes in order", () => {
+		const ws = fakeSocket();
+		const handle = createSurfaceSocket({
+			url: "ws://test",
+			onMessage: vi.fn(),
+			socketFactory: () => ws,
+		});
+
+		handle.send({ type: "subscribe", surfaceId: "a" });
+		handle.send({ type: "subscribe", surfaceId: "b" });
+		handle.send({ type: "invoke", surfaceId: "a", actionId: "x", payload: 1 });
+		ws.resolveOpen();
+
+		expect(ws.sent).toHaveLength(3);
+		expect(JSON.parse(ws.sent[0] ?? "")).toEqual({ type: "subscribe", surfaceId: "a" });
+		expect(JSON.parse(ws.sent[1] ?? "")).toEqual({ type: "subscribe", surfaceId: "b" });
+		expect(JSON.parse(ws.sent[2] ?? "")).toEqual({
+			type: "invoke",
+			surfaceId: "a",
+			actionId: "x",
+			payload: 1,
+		});
+	});
+});
diff --git a/src/adapters/ws/index.ts b/src/adapters/ws/index.ts
new file mode 100644
index 0000000..40eda2b
--- /dev/null
+++ b/src/adapters/ws/index.ts
@@ -0,0 +1,98 @@
+import type { SurfaceClientMessage, SurfaceServerMessage } from "@dispatch/ui-contract";
+import { nextBackoffMs, parseServerMessage, serialize } from "./logic";
+
+export interface WebSocketLike {
+	send(data: string): void;
+	close(): void;
+	onopen: (() => void) | null;
+	onmessage: ((ev: { data: string }) => void) | null;
+	onclose: ((ev: { code: number; reason: string }) => void) | null;
+}
+
+export interface SurfaceSocketOptions {
+	url: string;
+	onMessage: (msg: SurfaceServerMessage) => void;
+	onReopen?: () => void;
+	socketFactory?: (url: string) => WebSocketLike;
+}
+
+export interface SurfaceSocketHandle {
+	send(msg: SurfaceClientMessage): void;
+	close(): void;
+}
+
+export function createSurfaceSocket(opts: SurfaceSocketOptions): SurfaceSocketHandle {
+	const factory =
+		opts.socketFactory ?? ((url: string) => new WebSocket(url) as unknown as WebSocketLike);
+
+	let socket: WebSocketLike | null = null;
+	let disposed = false;
+	let reconnectAttempt = 0;
+	let reconnectTimer: ReturnType<typeof setTimeout> | null = null;
+	let isOpen = false;
+	const queue: string[] = [];
+
+	function connect(isReconnect: boolean): void {
+		socket = factory(opts.url);
+		isOpen = false;
+
+		socket.onopen = () => {
+			if (disposed) return;
+			isOpen = true;
+			reconnectAttempt = 0;
+			for (const raw of queue.splice(0)) {
+				socket?.send(raw);
+			}
+			if (isReconnect) {
+				opts.onReopen?.();
+			}
+		};
+
+		socket.onmessage = (ev) => {
+			if (disposed) return;
+			const msg = parseServerMessage(ev.data);
+			if (msg !== null) {
+				opts.onMessage(msg);
+			}
+		};
+
+		socket.onclose = () => {
+			if (disposed) return;
+			isOpen = false;
+			scheduleReconnect();
+		};
+	}
+
+	function scheduleReconnect(): void {
+		const delay = nextBackoffMs(reconnectAttempt);
+		reconnectAttempt++;
+		reconnectTimer = setTimeout(() => {
+			reconnectTimer = null;
+			if (disposed) return;
+			connect(true);
+		}, delay);
+	}
+
+	connect(false);
+
+	return {
+		send(msg: SurfaceClientMessage): void {
+			if (disposed) return;
+			const raw = serialize(msg);
+			if (isOpen) {
+				socket?.send(raw);
+			} else {
+				queue.push(raw);
+			}
+		},
+		close(): void {
+			disposed = true;
+			if (reconnectTimer !== null) {
+				clearTimeout(reconnectTimer);
+				reconnectTimer = null;
+			}
+			socket?.close();
+			socket = null;
+		},
+	};
+}
diff --git a/src/adapters/ws/logic.test.ts b/src/adapters/ws/logic.test.ts
new file mode 100644
index 0000000..62ae6a0
--- /dev/null
+++ b/src/adapters/ws/logic.test.ts
@@ -0,0 +1,195 @@
+import { describe, expect, it } from "vitest";
+import { nextBackoffMs, parseServerMessage, serialize } from "./logic";
+
+describe("serialize", () => {
+	it("serializes a subscribe message", () => {
+		const msg = { type: "subscribe" as const, surfaceId: "s1" };
+		expect(JSON.parse(serialize(msg))).toEqual(msg);
+	});
+
+	it("serializes an unsubscribe message", () => {
+		const msg = { type: "unsubscribe" as const, surfaceId: "s1" };
+		expect(JSON.parse(serialize(msg))).toEqual(msg);
+	});
+
+	it("serializes an invoke message with payload", () => {
+		const msg = { type: "invoke" as const, surfaceId: "s1", actionId: "toggle", payload: true };
+		expect(JSON.parse(serialize(msg))).toEqual(msg);
+	});
+
+	it("serializes an invoke message without payload", () => {
+		const msg = { type: "invoke" as const, surfaceId: "s1", actionId: "click" };
+		expect(JSON.parse(serialize(msg))).toEqual(msg);
+	});
+});
+
+describe("parseServerMessage", () => {
+	it("parses a catalog message", () => {
+		const data = JSON.stringify({
+			type: "catalog",
+			catalog: [{ id: "s1", region: "r", title: "S1" }],
+		});
+		const result = parseServerMessage(data);
+		expect(result).toEqual({
+			type: "catalog",
+			catalog: [{ id: "s1", region: "r", title: "S1" }],
+		});
+	});
+
+	it("parses a surface message", () => {
+		const data = JSON.stringify({
+			type: "surface",
+			spec: { id: "s1", region: "r", title: "S1", fields: [] },
+		});
+		const result = parseServerMessage(data);
+		expect(result).toEqual({
+			type: "surface",
+			spec: { id: "s1", region: "r", title: "S1", fields: [] },
+		});
+	});
+
+	it("parses an update message", () => {
+		const data = JSON.stringify({
+			type: "update",
+			update: {
+				surfaceId: "s1",
+				spec: { id: "s1", region: "r", title: "S1", fields: [] },
+			},
+		});
+		const result = parseServerMessage(data);
+		expect(result).toEqual({
+			type: "update",
+			update: {
+				surfaceId: "s1",
+				spec: { id: "s1", region: "r", title: "S1", fields: [] },
+			},
+		});
+	});
+
+	it("parses an error message with surfaceId", () => {
+		const data = JSON.stringify({ type: "error", surfaceId: "s1", message: "boom" });
+		const result = parseServerMessage(data);
+		expect(result).toEqual({ type: "error", surfaceId: "s1", message: "boom" });
+	});
+
+	it("parses an error message without surfaceId", () => {
+		const data = JSON.stringify({ type: "error", message: "global boom" });
+		const result = parseServerMessage(data);
+		expect(result).toEqual({ type: "error", message: "global boom" });
+	});
+
+	it("returns null for malformed JSON", () => {
+		expect(parseServerMessage("not json")).toBeNull();
+		expect(parseServerMessage("{broken")).toBeNull();
+		expect(parseServerMessage("")).toBeNull();
+	});
+
+	it("returns null for non-object JSON", () => {
+		expect(parseServerMessage("42")).toBeNull();
+		expect(parseServerMessage('"hello"')).toBeNull();
+		expect(parseServerMessage("null")).toBeNull();
+		expect(parseServerMessage("true")).toBeNull();
+		expect(parseServerMessage("[1,2,3]")).toBeNull();
+	});
+
+	it("returns null for unknown type", () => {
+		expect(parseServerMessage(JSON.stringify({ type: "unknown" }))).toBeNull();
+	});
+
+	it("returns null when type is missing", () => {
+		expect(parseServerMessage(JSON.stringify({ foo: "bar" }))).toBeNull();
+	});
+
+	it("returns null when type is not a string", () => {
+		expect(parseServerMessage(JSON.stringify({ type: 42 }))).toBeNull();
+	});
+
+	it("returns null for catalog with non-array catalog field", () => {
+		expect(parseServerMessage(JSON.stringify({ type: "catalog", catalog: "nope" }))).toBeNull();
+	});
+
+	it("returns null for surface with missing spec fields", () => {
+		expect(parseServerMessage(JSON.stringify({ type: "surface", spec: { id: "s1" } }))).toBeNull();
+	});
+
+	it("returns null for surface with non-object spec", () => {
+		expect(parseServerMessage(JSON.stringify({ type: "surface", spec: "nope" }))).toBeNull();
+	});
+
+	it("returns null for update with missing update field", () => {
+		expect(parseServerMessage(JSON.stringify({ type: "update" }))).toBeNull();
+	});
+
+	it("returns null for update with invalid spec", () => {
+		expect(
+			parseServerMessage(JSON.stringify({ type: "update", update: { surfaceId: "s1", spec: {} } })),
+		).toBeNull();
+	});
+
+	it("returns null for error with non-string message", () => {
+		expect(parseServerMessage(JSON.stringify({ type: "error", message: 42 }))).toBeNull();
+	});
+
+	it("returns null for error with invalid surfaceId type", () => {
+		expect(
+			parseServerMessage(JSON.stringify({ type: "error", surfaceId: 42, message: "boom" })),
+		).toBeNull();
+	});
+});
+
+describe("round-trip: parseServerMessage(serialize(...))", () => {
+	it("round-trips a subscribe message through serialize only", () => {
+		const msg = { type: "subscribe" as const, surfaceId: "s1" };
+		const wire = serialize(msg);
+		expect(JSON.parse(wire)).toEqual(msg);
+	});
+
+	it("round-trips an invoke message with payload", () => {
+		const msg = { type: "invoke" as const, surfaceId: "s1", actionId: "toggle", payload: false };
+		const wire = serialize(msg);
+		expect(JSON.parse(wire)).toEqual(msg);
+	});
+});
+
+describe("nextBackoffMs", () => {
+	it("returns a positive number", () => {
+		expect(nextBackoffMs(0)).toBeGreaterThan(0);
+	});
+
+	it("is capped at 30s + jitter (at most ~36s)", () => {
+		for (let i = 0; i < 100; i++) {
+			expect(nextBackoffMs(100)).toBeLessThanOrEqual(36_000);
+		}
+	});
+
+	it("starts around 500ms (±20% jitter)", () => {
+		for (let i = 0; i < 100; i++) {
+			const ms = nextBackoffMs(0);
+			expect(ms).toBeGreaterThanOrEqual(400);
+			expect(ms).toBeLessThanOrEqual(600);
+		}
+	});
+
+	it("grows exponentially with attempt", () => {
+		const averages = [0, 1, 2, 3].map((attempt) => {
+			let sum = 0;
+			for (let i = 0; i < 200; i++) {
+				sum += nextBackoffMs(attempt);
+			}
+			return sum / 200;
+		});
+		for (let i = 1; i < averages.length; i++) {
+			const prev = averages[i - 1];
+			if (prev === undefined) throw new Error("unreachable");
+			expect(averages[i]).toBeGreaterThan(prev);
+		}
+	});
+
+	it("treats negative attempt as 0", () => {
+		for (let i = 0; i < 50; i++) {
+			const ms = nextBackoffMs(-5);
+			expect(ms).toBeGreaterThanOrEqual(400);
+			expect(ms).toBeLessThanOrEqual(600);
+		}
+	});
+});
diff --git a/src/adapters/ws/logic.ts b/src/adapters/ws/logic.ts
new file mode 100644
index 0000000..83a5802
--- /dev/null
+++ b/src/adapters/ws/logic.ts
@@ -0,0 +1,91 @@
+import type {
+	CatalogMessage,
+	SurfaceClientMessage,
+	SurfaceErrorMessage,
+	SurfaceMessage,
+	SurfaceServerMessage,
+	SurfaceUpdateMessage,
+} from "@dispatch/ui-contract";
+
+const VALID_SERVER_TYPES = new Set(["catalog", "surface", "update", "error"]);
+
+/** Serialize a client message to a JSON string for the wire. */
+export function serialize(msg: SurfaceClientMessage): string {
+	return JSON.stringify(msg);
+}
+
+function isRecord(v: unknown): v is Record<string, unknown> {
+	return v !== null && typeof v === "object" && !Array.isArray(v);
+}
+
+/**
+ * Parse a raw server message string into a typed SurfaceServerMessage.
+ * Returns null for malformed JSON or shapes that don't match the protocol.
+ */
+export function parseServerMessage(data: string): SurfaceServerMessage | null {
+	let parsed: unknown;
+	try {
+		parsed = JSON.parse(data);
+	} catch {
+		return null;
+	}
+	if (!isRecord(parsed)) {
+		return null;
+	}
+	const t = parsed.type;
+	if (typeof t !== "string" || !VALID_SERVER_TYPES.has(t)) {
+		return null;
+	}
+	switch (t) {
+		case "catalog": {
+			if (!Array.isArray(parsed.catalog)) return null;
+			return { type: "catalog", catalog: parsed.catalog as CatalogMessage["catalog"] };
+		}
+		case "surface": {
+			const spec = parsed.spec;
+			if (!isRecord(spec)) return null;
+			if (typeof spec.id !== "string") return null;
+			if (typeof spec.region !== "string") return null;
+			if (typeof spec.title !== "string") return null;
+			if (!Array.isArray(spec.fields)) return null;
+			return { type: "surface", spec: spec as unknown as SurfaceMessage["spec"] };
+		}
+		case "update": {
+			const update = parsed.update;
+			if (!isRecord(update)) return null;
+			if (typeof update.surfaceId !== "string") return null;
+			const spec = update.spec;
+			if (!isRecord(spec)) return null;
+			if (typeof spec.id !== "string") return null;
+			if (typeof spec.region !== "string") return null;
+			if (typeof spec.title !== "string") return null;
+			if (!Array.isArray(spec.fields)) return null;
+			return { type: "update", update: update as unknown as SurfaceUpdateMessage["update"] };
+		}
+		case "error": {
+			if (typeof parsed.message !== "string") return null;
+			const surfaceId = parsed.surfaceId;
+			if (surfaceId !== undefined && typeof surfaceId !== "string") return null;
+			const msg: SurfaceErrorMessage =
+				surfaceId !== undefined
+					? { type: "error", surfaceId, message: parsed.message }
+					: { type: "error", message: parsed.message };
+			return msg;
+		}
+		default:
+			return null;
+	}
+}
+
+/**
+ * Bounded exponential backoff with jitter.
+ * Base: 500ms, doubles each attempt, caps at 30s, adds ±20% jitter.
+ */
+export function nextBackoffMs(attempt: number): number {
+	const base = 500;
+	const max = 30_000;
+	const exponential = base * 2 ** Math.max(0, attempt);
+	const capped = Math.min(exponential, max);
+	const jitter = 0.8 + Math.random() * 0.4;
+	return Math.round(capped * jitter);
+}