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.

1 files changed, 82 insertions, 0 deletions

diff --git a/src/core/protocol/reducer.ts b/src/core/protocol/reducer.ts
new file mode 100644
index 0000000..992a918
--- /dev/null
+++ b/src/core/protocol/reducer.ts
@@ -0,0 +1,82 @@
+import type {
+	InvokeMessage,
+	SubscribeMessage,
+	SurfaceServerMessage,
+	UnsubscribeMessage,
+} from "@dispatch/ui-contract";
+import type { ProtocolResult, ProtocolState } from "./types";
+
+/** The initial protocol state: empty catalog, no subscriptions, no error. */
+export function initialState(): ProtocolState {
+	return {
+		catalog: [],
+		subscriptions: new Map(),
+		lastError: null,
+	};
+}
+
+/** Fold an inbound server message into the next protocol state. */
+export function applyServerMessage(state: ProtocolState, msg: SurfaceServerMessage): ProtocolState {
+	switch (msg.type) {
+		case "catalog":
+			return { ...state, catalog: msg.catalog };
+
+		case "surface": {
+			const surfaceId = msg.spec.id;
+			if (!state.subscriptions.has(surfaceId)) return state;
+			const subs = new Map(state.subscriptions);
+			subs.set(surfaceId, msg.spec);
+			return { ...state, subscriptions: subs };
+		}
+
+		case "update": {
+			const surfaceId = msg.update.surfaceId;
+			if (!state.subscriptions.has(surfaceId)) return state;
+			const subs = new Map(state.subscriptions);
+			subs.set(surfaceId, msg.update.spec);
+			return { ...state, subscriptions: subs };
+		}
+
+		case "error":
+			return { ...state, lastError: msg };
+	}
+}
+
+/**
+ * Subscribe to a surface. Idempotent: if already subscribed, returns the same
+ * state with no outgoing message.
+ */
+export function subscribe(state: ProtocolState, surfaceId: string): ProtocolResult {
+	if (state.subscriptions.has(surfaceId)) {
+		return { state, outgoing: [] };
+	}
+	const subs = new Map(state.subscriptions);
+	subs.set(surfaceId, null);
+	const outgoing: SubscribeMessage = { type: "subscribe", surfaceId };
+	return { state: { ...state, subscriptions: subs }, outgoing: [outgoing] };
+}
+
+/**
+ * Unsubscribe from a surface. Drops the local spec and emits one unsubscribe.
+ * If not subscribed, returns the same state with no outgoing.
+ */
+export function unsubscribe(state: ProtocolState, surfaceId: string): ProtocolResult {
+	if (!state.subscriptions.has(surfaceId)) {
+		return { state, outgoing: [] };
+	}
+	const subs = new Map(state.subscriptions);
+	subs.delete(surfaceId);
+	const outgoing: UnsubscribeMessage = { type: "unsubscribe", surfaceId };
+	return { state: { ...state, subscriptions: subs }, outgoing: [outgoing] };
+}
+
+/** Invoke a field's action on a surface. Emits an InvokeMessage; no state change. */
+export function invoke(
+	state: ProtocolState,
+	surfaceId: string,
+	actionId: string,
+	payload?: unknown,
+): ProtocolResult {
+	const outgoing: InvokeMessage = { type: "invoke", surfaceId, actionId, payload };
+	return { state, outgoing: [outgoing] };
+}