refactor: frame bus + pipe mode tests (unified frame pipe follow-ups) (#725)

* test(runtimed): add pipe mode integration tests

Cover the unified pipe code paths that had zero test coverage:
- Sync frames forwarded with correct type byte prefix
- Only valid frame types piped (never Request/Response)
- Response frames consumed by request/response cycle, not piped
- Frame ordering preserved across rapid mutations

* refactor: replace webview.emit fan-out with in-memory frame bus

Introduce notebook-frame-bus.ts — a module-level pub/sub that replaces
the webview.emit('notebook:broadcast') and webview.emit('notebook:presence')
fan-out pattern. Subscribers now receive payloads via direct function
call instead of async Tauri event dispatch.

- useAutomergeNotebook: calls emitBroadcast/emitPresence instead of
  webview.emit after WASM demux
- useDaemonKernel: subscribeBroadcast() replaces webview.listen
- useEnvProgress: subscribeBroadcast() replaces webview.listen
- usePresence: subscribePresence() replaces webview.listen

Only notebook:frame remains as a Tauri webview event (the relay pipe).
All downstream dispatch is synchronous and in-memory.

* test: strengthen pipe mode tests per Codex review

- Rename test_pipe_mode_forwards_broadcast_frames to
  test_pipe_mode_only_pipes_allowed_frame_types — honest about what
  it checks (type-byte filter, not broadcast-specific forwarding)
- Add comment noting broadcast frames require a kernel (E2E coverage)
- test_pipe_mode_preserves_frame_order now verifies:
  - No duplicate frames (coalescing would violate ordering contract)
  - Third client convergence (all 3 cells with correct sources)

* docs: note ordering test limitation per Codex review

Author: rgbkrk

apps/notebook/src/hooks/useAutomergeNotebook.ts

@@ -19,6 +19,7 @@ import {
   updateNotebookCells,
   useNotebookCells,
 } from "../lib/notebook-cells";
+import { emitBroadcast, emitPresence } from "../lib/notebook-frame-bus";
 import {
   notifyMetadataChanged,
   setNotebookHandle,
@@ -216,8 +217,8 @@ export function useAutomergeNotebook() {
     // one event. The WASM handle.receive_frame() demuxes by the first byte,
     // applies sync internally, and returns typed FrameEvent JSON.
     //
-    // Broadcasts are re-emitted as "notebook:broadcast" for backward compat
-    // with useDaemonKernel and useEnvProgress (they listen independently).
+    // Broadcasts and presence are dispatched via the frame bus (in-memory
+    // pub/sub) to useDaemonKernel, useEnvProgress, and usePresence.
     const unlistenFrame = webview.listen<number[]>(
       "notebook:frame",
       async (event) => {
@@ -264,21 +265,14 @@ export function useAutomergeNotebook() {
                 break;
               }
               case "broadcast": {
-                // Re-emit as "notebook:broadcast" for useDaemonKernel/useEnvProgress
-                // backward compat. They listen independently and expect JSON payloads.
                 if (frameEvent.payload) {
-                  webview
-                    .emit("notebook:broadcast", frameEvent.payload)
-                    .catch(() => {});
+                  emitBroadcast(frameEvent.payload);
                 }
                 break;
               }
               case "presence": {
-                // Re-emit for usePresence hook
                 if (frameEvent.payload) {
-                  webview
-                    .emit("notebook:presence", frameEvent.payload)
-                    .catch(() => {});
+                  emitPresence(frameEvent.payload);
                 }
                 break;
               }