fix(preload): one shared ipcRenderer listener for per-pane channels

[Juliusolsson05]

Closes #1015.

Why

The dev app warned at every startup:

MaxListenersExceededWarning: 11 goal-loop:changed listeners added to [IpcRenderer] (and the same for dictation:stream-transcript).

It is not a leak. Every mounted GoalLoopPane and composer subscribes once and cleans up. But each subscription was its own ipcRenderer listener, so 11 panes crossed Node's default of 10. A permanent false alarm hides the next real leak. Raising MaxListeners would silence exactly that alarm.

Change

• subscribeShared(channel, cb) in preload/api/ipc.ts:
  • one relay listener per channel, fanned out to a set of subscribers;
  • the relay is removed with the last subscriber;
  • each subscriber is isolated, so one pane that throws doesn't stop the others hearing the event (separate EventEmitter listeners didn't guarantee that either).
• goal-loop:changed and dictation:stream-transcript use it.
• LSP diagnostics keeps its own permanent listener on purpose: code blocks mount and unmount while scrolling, and its comment explains why it must not churn.

Tests

src/preload/api/sharedListeners.test.ts uses a real EventEmitter as ipcRenderer, for both channels:

• 12 subscribers create 1 listener, and every subscriber gets every event;
• the last unsubscribe removes the listener, and no MaxListeners warning is emitted;
• a subscriber that throws doesn't starve the others.

All four cases fail on main. The goal-loop and dictation renderer suites pass (18), and npx tsc -b is clean.

🤖 Generated with Claude Code

[Juliusolsson05]

Review (CHANGES REQUESTED) addressed in ac0878c6:

• MEDIUM: record-session:started, record-session:stopping, tldr:changed and goal:changed use subscribeShared. The shared-listener test now covers all six channels, and each fails on main with 12 listeners.
• LOW: the warning spy now waits a tick (Node emits the warning on process.nextTick), so the check can fail.
• NIT: the comment explains why LSP keeps its own permanent listener.

preload: 15/15, and tsc is clean.