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, 8 insertions, 5 deletions

diff --git a/packages/core/src/chunks/append.ts b/packages/core/src/chunks/append.ts
index baccd10..4bc6b5f 100644
--- a/packages/core/src/chunks/append.ts
+++ b/packages/core/src/chunks/append.ts
@@ -6,7 +6,7 @@
  * backend (agent + agent-manager) and the frontend store call this helper
  * so the wire format stays in lockstep across the boundary.
  *
- * Open/close rules — see plan-chunk-refactor.md for the full table.
+ * Open/close rules — see notes/plan-chunk-refactor.md for the full table.
  *
  *  | Chunk         | Opens on                                                        | Coalesces                                                  |
  *  |---------------|-----------------------------------------------------------------|------------------------------------------------------------|
@@ -35,9 +35,10 @@
  *                       (no unsealed thinking chunk) are dropped.
  *
  * Ignored events:
- *  - `status`, `done`, `usage`, `task-list-update`, `tab-created`,
- *    `message-queued`, `message-consumed`, `message-cancelled` — these are
- *    control / lifecycle events, not message content.
+ *  - `status`, `turn-start`, `turn-sealed`, `done`, `usage`,
+ *    `task-list-update`, `tab-created`, `message-queued`, `message-consumed`,
+ *    `message-cancelled` — these are control / lifecycle events, not message
+ *    content.
  */
 
 import type {
@@ -200,6 +201,8 @@ export function appendEventToChunks(chunks: Chunk[], event: AgentEvent): void {
 
 		// Lifecycle / control events — no chunk emitted.
 		case "status":
+		case "turn-start":
+		case "turn-sealed":
 		case "done":
 		case "usage":
 		case "task-list-update":
@@ -251,7 +254,7 @@ export interface SystemEventLike {
  * in flight*. (When a turn IS in flight, the caller should instead use
  * `appendEventToChunks` against the in-flight message's chunks directly.)
  *
- * Routing rules (from plan-chunk-refactor.md):
+ * Routing rules (from notes/plan-chunk-refactor.md):
  *
  *  1. Most recent message is `role: "system"` → append a `system` chunk
  *     to it. (Note: a second consecutive system event creates a second