feat(chat): cancel a queued steering message from the UI

Implements the frontend for the cancel-queued-message backend feature
(transport-contract 0.23.0 → 0.24.0, ADDITIVE). While a turn is generating
and a user message is queued (awaiting steering delivery), the user can now
cancel a single queued message by id so it never runs.

- Consume the contract: regenerate the .dispatch/transport-contract.reference.md
  mirror to 0.24.0; add chat.queue.cancel to the exhaustive WsClientMessage
  guard (core/wire/conformance.ts) + its test.
- ChatTransport port accepts ChatQueueCancelMessage; cancelQueuedMessage(id)
  on the chat store + app store sends chat.queue.cancel { conversationId,
  messageId } (fire-and-forget, idempotent — the server no-ops an already-
  drained/unknown id).
- UI: a × cancel affordance per queued row in MessageQueueList.svelte,
  threaded through SurfaceView's onCancelQueuedMessage (dispatched on
  rendererId, never the surface id) and wired to store.cancelQueuedMessage.
  Optimistic removal (pure selectVisibleMessages/reconcileCancelledIds in
  logic/message-queue.ts) hides the row on click and reconciles from the
  message-queue surface's post-cancel snapshot. No new event handling — the
  existing surface subscription reflects the result.

Tests: +12 (chat store cancel op ×3, optimistic-removal logic ×11 already
existed pattern, MessageQueueList component cancel behavior ×9).

Repo fix: package.json + bun.lock were still pinning file:../dispatch-backend/...
(stale from the dispatch-backend → backend rename) — corrected to
file:../backend/packages/...; bun install now resolves natively with no
worktree symlink hack.

typecheck 0/0, 1140 tests green (run twice), biome clean, build OK.

2 files changed, 4 insertions, 0 deletions

diff --git a/src/core/wire/conformance.test.ts b/src/core/wire/conformance.test.ts
index 0d955d5..2b98ca6 100644
--- a/src/core/wire/conformance.test.ts
+++ b/src/core/wire/conformance.test.ts
@@ -212,6 +212,7 @@ describe("classifies every WsClientMessage type", () => {
       { type: "chat.subscribe" as const, conversationId: "c1" },
       { type: "chat.unsubscribe" as const, conversationId: "c1" },
       { type: "chat.queue" as const, conversationId: "c1", text: "steer" },
+      { type: "chat.queue.cancel" as const, conversationId: "c1", messageId: "m1" },
     ];
     const labels = msgs.map(assertWsClientMessageExhaustive);
     expect(labels).toEqual([
@@ -222,6 +223,7 @@ describe("classifies every WsClientMessage type", () => {
       "chat.subscribe",
       "chat.unsubscribe",
       "chat.queue",
+      "chat.queue.cancel",
     ]);
   });
 });
diff --git a/src/core/wire/conformance.ts b/src/core/wire/conformance.ts
index bfa67dc..16558cd 100644
--- a/src/core/wire/conformance.ts
+++ b/src/core/wire/conformance.ts
@@ -116,6 +116,8 @@ export function assertWsClientMessageExhaustive(msg: WsClientMessage): string {
       return "chat.unsubscribe";
     case "chat.queue":
       return "chat.queue";
+    case "chat.queue.cancel":
+      return "chat.queue.cancel";
     default:
       return msg satisfies never;
   }