feat(message-queue): per-conversation queue + steering injection

A per-conversation message queue (new message-queue extension) holds user
messages enqueued while a turn generates; delivered mid-turn as steering at the
tool-result boundary (or carried to a new turn if no tool call fires).

- kernel: RunTurnInput.drainSteering callback (generic; kernel stays pure)
- wire 0.7.0->0.8.0: QueuedMessage, QueuePayload, TurnSteeringEvent (additive)
- transport-contract 0.11.0->0.12.0: POST /conversations/:id/queue + chat.queue WS op
- message-queue ext: queue state + per-conversation custom surface (rendererId message-queue)
- session-orchestrator: enqueue facade + drainSteering wiring + post-seal carry
- transport-http/ws: queue endpoint + chat.queue op (fixes WsClientMessage exhaustive switch)
- host-bin: register message-queue

1043 vitest + 199 transport bun pass; tsc/biome clean; boot smoke clean.
FE courier: frontend-message-queue-handoff.md.

1 files changed, 27 insertions, 0 deletions

diff --git a/packages/message-queue/src/index.ts b/packages/message-queue/src/index.ts
new file mode 100644
index 0000000..79bbb25
--- /dev/null
+++ b/packages/message-queue/src/index.ts
@@ -0,0 +1,27 @@
+/**
+ * @dispatch/message-queue — the per-conversation message queue for mid-turn
+ * steering. Owns the queue state + a per-conversation `custom` surface; the
+ * session-orchestrator drains it via `messageQueueHandle`. The queue is
+ * transient (in-memory, per-conversation, only meaningful during generation);
+ * the surface is the ONLY way the frontend reads queue state.
+ */
+
+export type { QueuedMessage, QueuePayload } from "@dispatch/wire";
+export { extension, manifest } from "./extension.js";
+export {
+	buildQueueSpec,
+	combine,
+	drain,
+	enqueue,
+	getQueue,
+	MESSAGE_QUEUE_RENDERER_ID,
+	MESSAGE_QUEUE_SURFACE_ID,
+	type MessageQueueState,
+	type QueueDeps,
+} from "./pure.js";
+export {
+	createMessageQueueService,
+	type MessageQueueDeps,
+	type MessageQueueService,
+	messageQueueHandle,
+} from "./service.js";