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, 3 insertions, 0 deletions

diff --git a/GLOSSARY.md b/GLOSSARY.md
index 61a555d..a1ef33c 100644
--- a/GLOSSARY.md
+++ b/GLOSSARY.md
@@ -34,6 +34,9 @@
 | **hook** | A typed extension point. **event** = fire-and-forget, N listeners, error-isolated. **filter** = ordered value-in→value-out chain, in-band. | callback (when meaning a hook), listener |
 | **service** | A single-responder request/response capability fetched via a typed handle. NOT a hook. | — |
 | **dispatch policy** | `{ maxConcurrent, eager }` controlling how the turn loop runs a step's tool calls. | — |
+| **message queue** | The per-conversation buffer of user messages a client (FE or CLI) enqueues while a turn is GENERATING, awaiting mid-turn steering delivery. Owned by the `message-queue` extension; exposed to the frontend as a per-conversation `custom` surface (`rendererId: "message-queue"`, `QueuePayload`). Enqueuing when no turn is active starts a new turn instead. | steering queue, pending queue |
+| **steering** | A user message injected into an in-flight turn at the tool-result boundary (drawn from the message queue), so the model sees it alongside the tool results and can adjust course mid-turn. Emitted on the chat stream as a `steering` `AgentEvent`. If the turn ENDS with a non-empty queue (no tool call fired), the queue is instead carried into a NEW turn as its opening prompt. | interjection, mid-turn input |
+| **queued message** | An item in the message queue: `{ id, text, queuedAt }`. The unit the frontend renders in the queue surface. | pending message |
 | **reconcile** | The pure function run on load that repairs a partial/interrupted turn into a valid history. | recover, repair |
 | **session-orchestrator** | The core extension that drives a turn: load history → resolve provider/tools → call `runTurn` → persist. | — |
 | **conversation-store** | The core extension persisting the append-only turn/chunk log. | message store |