feat(chunks): chunk-native frontend store with turn-sealed reconcile + per-chunk eviction

Replace the stored ChatMessage[] with a chunk-native model: tab.chunks (sealed
ChunkRow[]) + tab.live (transient in-flight turn buffer) + derived tab.renderGroups.
This enables per-chunk eviction (trimming WITHIN a large turn) and raw-chunk
pagination (loadOlderChunks), removing the whole-message eviction limitation.

Backend:
- Emit turn-start/turn-sealed around each turn; expose currentTurnId in the status
  snapshot. turn-sealed fires after the durable write (status:idle fires before it).
- New GET /tabs/:id/chunks raw paginated endpoint (limit/before).
- Wrap appendChunks in a single SQLite transaction.

Frontend:
- turn-sealed drives a turn-aware reconcile that folds the sealed turn into chunks
  while preserving a concurrent newer in-flight turn and pending queued messages;
  deferred while the user is scrolled up.
- Stable turn-scoped render keys (${turnId}:${role}:${n}) avoid remount/flash.

Reconcile correctness (three review passes):
- preserve a concurrent newer turn when an earlier deferred reconcile flushes;
- keep optimistic queued user messages (no loss);
- turn-start backfill skips pending queued rows and tags only the turn initiator;
- bind consumed interrupt messages to the in-flight turn so they collapse on seal
  (no lingering/duplicated bubble).

Tests: chat-store reconcile/eviction/pagination suite; api chunks endpoint + events.

1 files changed, 24 insertions, 0 deletions

diff --git a/packages/api/src/routes/tabs.ts b/packages/api/src/routes/tabs.ts
index e9265ec..b1e9659 100644
--- a/packages/api/src/routes/tabs.ts
+++ b/packages/api/src/routes/tabs.ts
@@ -93,6 +93,30 @@ tabsRoutes.get("/:id/messages", (c) => {
 	return c.json({ messages, total, oldestSeq });
 });
 
+// Raw chunk window for a tab — the chunk-native frontend's load/paginate
+// source. Same `limit`/`before` chunk-`seq` windowing as `/messages`, but
+// returns the flat `ChunkRow[]` WITHOUT server-side grouping (the frontend
+// groups for render and evicts/paginates on the flat list). Dedupe on the
+// client by `seq` when overlap-fetching.
+tabsRoutes.get("/:id/chunks", (c) => {
+	const id = c.req.param("id");
+	const limitRaw = c.req.query("limit");
+	const beforeRaw = c.req.query("before");
+	const limit = limitRaw !== undefined ? Number(limitRaw) : undefined;
+	const before = beforeRaw !== undefined ? Number(beforeRaw) : undefined;
+	const options =
+		limit !== undefined || before !== undefined
+			? {
+					...(limit !== undefined && Number.isFinite(limit) ? { limit } : {}),
+					...(before !== undefined && Number.isFinite(before) ? { before } : {}),
+				}
+			: undefined;
+	const chunks = getChunksForTab(id, options);
+	const oldestSeq = chunks.length > 0 ? (chunks[0]?.seq ?? null) : null;
+	const total = getTotalChunkCount(id);
+	return c.json({ chunks, total, oldestSeq });
+});
+
 tabsRoutes.patch("/:id", async (c) => {
 	const id = c.req.param("id");
 	const body = await c.req.json<{