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.

1 files changed, 61 insertions, 0 deletions

diff --git a/src/features/surface-host/logic/message-queue.ts b/src/features/surface-host/logic/message-queue.ts
index 79707a5..7a3653e 100644
--- a/src/features/surface-host/logic/message-queue.ts
+++ b/src/features/surface-host/logic/message-queue.ts
@@ -43,3 +43,64 @@ export function parseMessageQueuePayload(payload: unknown): MessageQueueData | n
 
 /** The `rendererId` the message-queue extension's `custom` surface field uses. */
 export const MESSAGE_QUEUE_RENDERER_ID = "message-queue";
+
+/**
+ * Optimistic-removal view-model for the queue list.
+ *
+ * The `chat.queue.cancel` op is fire-and-forget + idempotent: success is
+ * confirmed by the `message-queue` SURFACE updating (the cancelled message
+ * leaves the snapshot), not by a reply. To avoid a flash of the row lingering
+ * for a round-trip, the renderer hides a row the instant the user clicks cancel
+ * (tracking the cancelled id locally), then reconciles when the surface pushes
+ * the post-cancel snapshot. These two pure helpers drive that — the component
+ * holds the cancelled-id set as a thin `$state` wrapper and delegates all
+ * decisions here.
+ */
+
+/**
+ * The messages the renderer should show: the surface snapshot MINUS any
+ * optimistically-cancelled ids (a cancel whose surface confirmation hasn't
+ * arrived yet). Pure — no mutation of inputs.
+ */
+export function selectVisibleMessages(
+  messages: readonly QueuedMessage[],
+  cancelledIds: ReadonlySet<string>,
+): readonly QueuedMessage[] {
+  if (cancelledIds.size === 0) return messages;
+  return messages.filter((m) => !cancelledIds.has(m.id));
+}
+
+/**
+ * Reconcile the cancelled-id set against a NEW surface snapshot: keep only the
+ * ids that are STILL queued (the cancel is pending — its surface confirmation
+ * hasn't landed). Drop ids that have left the snapshot: the server confirmed
+ * the removal (or the message drained as steering / the queue cleared), so the
+ * optimistic hide is no longer needed. This keeps the set bounded — it never
+ * outlives the rows it tracks. Pure — returns a NEW set (callers assign it to
+ * the reactive `$state`).
+ */
+export function reconcileCancelledIds(
+  messages: readonly QueuedMessage[],
+  cancelledIds: ReadonlySet<string>,
+): ReadonlySet<string> {
+  if (cancelledIds.size === 0) return EMPTY_STRING_SET;
+  const stillQueued = new Set<string>();
+  for (const m of messages) {
+    if (cancelledIds.has(m.id)) stillQueued.add(m.id);
+  }
+  // Same set back → return the input identity so the component's `$state` setter
+  // sees no change (avoids a spurious reactive cycle).
+  if (stillQueued.size === cancelledIds.size) {
+    let same = true;
+    for (const id of cancelledIds) {
+      if (!stillQueued.has(id)) {
+        same = false;
+        break;
+      }
+    }
+    if (same) return cancelledIds;
+  }
+  return stillQueued;
+}
+
+const EMPTY_STRING_SET: ReadonlySet<string> = new Set<string>();