feat(chat): message queue + steering — mid-turn injection at tool-result boundaries

Consume the message-queue + steering handoff ([email protected], [email protected]).
Re-pinned file: deps + re-mirrored .dispatch/*.reference.md.

- fold steering AgentEvent into the transcript as a provisional user bubble
  (after the tool-result it followed; no de-dup — the queue surface carried it)
- add rendererId: "message-queue" custom renderer (pure parser + MessageQueueList)
  rendered as a compact panel above the Composer (hidden when queue is empty)
- add ChatStore.queueMessage / AppStore.queueMessage — sends chat.queue WS op
  (trim/validate non-empty; auto-starts a turn if idle)
- Composer switches to chat.queue while generating (button → Queue, placeholder
  → Steer the conversation...)
- exhaustiveness guards updated for steering + chat.queue
- carry-to-new-turn needs no special handling (normal new turn)

664 tests green.

4 files changed, 118 insertions, 0 deletions

diff --git a/src/features/surface-host/logic/message-queue.test.ts b/src/features/surface-host/logic/message-queue.test.ts
new file mode 100644
index 0000000..ce078d9
--- /dev/null
+++ b/src/features/surface-host/logic/message-queue.test.ts
@@ -0,0 +1,48 @@
+import type { QueuedMessage } from "@dispatch/wire";
+import { describe, expect, it } from "vitest";
+import { parseMessageQueuePayload } from "./message-queue";
+
+const msg = (id: string, text: string, queuedAt = 1_700_000_000_000): QueuedMessage => ({
+	id,
+	text,
+	queuedAt,
+});
+
+describe("parseMessageQueuePayload", () => {
+	it("parses a well-formed payload with messages", () => {
+		const data = parseMessageQueuePayload({
+			messages: [msg("m1", "steer left"), msg("m2", "actually, go right")],
+		});
+		expect(data).toEqual({
+			messages: [msg("m1", "steer left"), msg("m2", "actually, go right")],
+		});
+	});
+
+	it("parses an empty-messages payload (queue is empty)", () => {
+		expect(parseMessageQueuePayload({ messages: [] })).toEqual({ messages: [] });
+	});
+
+	it("preserves message order", () => {
+		const data = parseMessageQueuePayload({
+			messages: [msg("a", "first"), msg("b", "second"), msg("c", "third")],
+		});
+		expect(data?.messages.map((m) => m.id)).toEqual(["a", "b", "c"]);
+	});
+
+	it.each([
+		["null", null],
+		["a number", 7],
+		["a string", "nope"],
+		["missing messages key", { foo: [] }],
+		["messages not an array", { messages: "x" }],
+		["entry not an object", { messages: ["x"] }],
+		["entry missing id", { messages: [{ text: "x", queuedAt: 1 }] }],
+		["entry with non-string id", { messages: [{ id: 1, text: "x", queuedAt: 1 }] }],
+		["entry missing text", { messages: [{ id: "m1", queuedAt: 1 }] }],
+		["entry with non-string text", { messages: [{ id: "m1", text: 1, queuedAt: 1 }] }],
+		["entry missing queuedAt", { messages: [{ id: "m1", text: "x" }] }],
+		["entry with non-finite queuedAt", { messages: [msg("m1", "x", Number.NaN)] }],
+	])("returns null for invalid payload: %s", (_label, payload) => {
+		expect(parseMessageQueuePayload(payload)).toBeNull();
+	});
+});
diff --git a/src/features/surface-host/logic/message-queue.ts b/src/features/surface-host/logic/message-queue.ts
new file mode 100644
index 0000000..a8e1567
--- /dev/null
+++ b/src/features/surface-host/logic/message-queue.ts
@@ -0,0 +1,45 @@
+import type { QueuedMessage } from "@dispatch/wire";
+
+/**
+ * Pure parser for the `rendererId: "message-queue"` custom-field payload.
+ *
+ * The message-queue extension's per-conversation surface emits ONE `custom`
+ * field with `rendererId: "message-queue"` and `payload: QueuePayload`
+ * (`{ messages: QueuedMessage[] }` — the current queue snapshot). This parser
+ * validates the untyped `payload: unknown` at the network seam so a
+ * hostile/partial payload can never crash the renderer (graceful skip → null).
+ *
+ * Empty `messages` is a valid, parseable state (the queue is empty — nothing to
+ * render); the caller hides the panel. Null is returned only for a malformed
+ * payload shape.
+ */
+export interface MessageQueueData {
+	readonly messages: readonly QueuedMessage[];
+}
+
+function isQueuedMessage(v: unknown): v is QueuedMessage {
+	if (typeof v !== "object" || v === null) return false;
+	const o = v as Record<string, unknown>;
+	return (
+		typeof o.id === "string" &&
+		typeof o.text === "string" &&
+		typeof o.queuedAt === "number" &&
+		Number.isFinite(o.queuedAt)
+	);
+}
+
+export function parseMessageQueuePayload(payload: unknown): MessageQueueData | null {
+	if (typeof payload !== "object" || payload === null) return null;
+	const obj = payload as Record<string, unknown>;
+	const raw = obj.messages;
+	if (!Array.isArray(raw)) return null;
+	const messages: QueuedMessage[] = [];
+	for (const entry of raw) {
+		if (!isQueuedMessage(entry)) return null;
+		messages.push(entry);
+	}
+	return { messages };
+}
+
+/** The `rendererId` the message-queue extension's `custom` surface field uses. */
+export const MESSAGE_QUEUE_RENDERER_ID = "message-queue";
diff --git a/src/features/surface-host/ui/MessageQueueList.svelte b/src/features/surface-host/ui/MessageQueueList.svelte
new file mode 100644
index 0000000..12de970
--- /dev/null
+++ b/src/features/surface-host/ui/MessageQueueList.svelte
@@ -0,0 +1,22 @@
+<script lang="ts">
+	import { parseMessageQueuePayload } from "../logic/message-queue";
+
+	let { payload }: { readonly payload: unknown } = $props();
+
+	// Parse defensively; an unparseable payload yields null → render nothing
+	// (graceful skip, per the custom-field contract).
+	const data = $derived(parseMessageQueuePayload(payload));
+</script>
+
+{#if data !== null && data.messages.length > 0}
+	<ul class="flex flex-col gap-1 text-sm">
+		{#each data.messages as msg (msg.id)}
+			<li class="rounded-box bg-base-200 px-3 py-2">
+				<p class="whitespace-pre-wrap">{msg.text}</p>
+				<time class="text-xs opacity-50" datetime={new Date(msg.queuedAt).toISOString()}>
+					{new Date(msg.queuedAt).toLocaleTimeString()}
+				</time>
+			</li>
+		{/each}
+	</ul>
+{/if}
diff --git a/src/features/surface-host/ui/SurfaceView.svelte b/src/features/surface-host/ui/SurfaceView.svelte
index 24be8b8..e5f807a 100644
--- a/src/features/surface-host/ui/SurfaceView.svelte
+++ b/src/features/surface-host/ui/SurfaceView.svelte
@@ -2,6 +2,7 @@
 	import type { InvokeMessage, SurfaceSpec } from "@dispatch/ui-contract";
 	import { groupRenderFields, planSurface } from "../logic/plan";
 	import Button from "./Button.svelte";
+	import MessageQueueList from "./MessageQueueList.svelte";
 	import Number from "./Number.svelte";
 	import Progress from "./Progress.svelte";
 	import Selector from "./Selector.svelte";
@@ -40,6 +41,8 @@
 			     unknown ids gracefully render nothing. -->
 			{#if group.field.rendererId === "table"}
 				<SurfaceTable payload={group.field.payload} />
+			{:else if group.field.rendererId === "message-queue"}
+				<MessageQueueList payload={group.field.payload} />
 			{/if}
 		{/if}
 	{/each}