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, 30 insertions, 2 deletions

diff --git a/packages/api/tests/agent-manager.test.ts b/packages/api/tests/agent-manager.test.ts
index 6b016db..4415bbb 100644
--- a/packages/api/tests/agent-manager.test.ts
+++ b/packages/api/tests/agent-manager.test.ts
@@ -340,15 +340,43 @@ describe("AgentManager", () => {
 		await manager.processMessage("tab-1", "test");
 
 		expect(events.length).toBeGreaterThan(0);
-		expect(events[0]).toMatchObject({ type: "status", status: "running" });
+		// A turn now opens with `turn-start`, immediately followed by the
+		// agent's `status: running`.
+		expect(events[0]).toMatchObject({ type: "turn-start" });
+		expect(events[1]).toMatchObject({ type: "status", status: "running" });
 
+		// A turn now closes with `turn-sealed` (emitted after the DB write, which
+		// is after the agent's final `status: idle`).
 		const lastEvent = events[events.length - 1];
-		expect(lastEvent).toMatchObject({ type: "status", status: "idle" });
+		expect(lastEvent).toMatchObject({ type: "turn-sealed" });
+		expect(events.some((e) => e.type === "status" && e.status === "idle")).toBe(true);
 
 		const doneEvent = events.find((e) => e.type === "done");
 		expect(doneEvent).toBeDefined();
 	});
 
+	it("emits a turn-start with a turnId before any content event", async () => {
+		const manager = new AgentManager();
+		const events: AgentEvent[] = [];
+		manager.onEvent((event) => {
+			events.push(event);
+		});
+
+		await manager.processMessage("tab-turnstart", "go");
+
+		const turnStartIdx = events.findIndex((e) => e.type === "turn-start");
+		expect(turnStartIdx).toBeGreaterThanOrEqual(0);
+		const turnStart = events[turnStartIdx] as Extract<AgentEvent, { type: "turn-start" }>;
+		expect(typeof turnStart.turnId).toBe("string");
+		expect(turnStart.turnId.length).toBeGreaterThan(0);
+
+		// Must precede the first content delta.
+		const firstContentIdx = events.findIndex(
+			(e) => e.type === "text-delta" || e.type === "reasoning-delta",
+		);
+		expect(firstContentIdx).toBeGreaterThan(turnStartIdx);
+	});
+
 	it("emits text-delta events during processMessage", async () => {
 		const manager = new AgentManager();
 		const events: AgentEvent[] = [];