fix(overlay): drive timing review playhead from mpv playback position

- Preview session observes time-pos, waits for loaded media before seeking, and acks each command
- Forward positions to the modal over media-timing-review:preview-position
- Cursor follows the real position, pauses while buffering, and stays at the clip end when done
- Replace the fixed end-of-clip timer with a 15s stall timeout
- Document preview cursor behavior and ownership in docs

src/shared/ipc/contracts.ts

@@ -167,6 +167,7 @@ export const IPC_CHANNELS = {
     youtubePickerOpen: 'youtube:picker-open',
     mediaTimingReviewOpen: 'media-timing-review:open',
     mediaTimingReviewPreviewEnded: 'media-timing-review:preview-ended',
+    mediaTimingReviewPreviewPosition: 'media-timing-review:preview-position',
     youtubePickerCancel: 'youtube:picker-cancel',
     playlistBrowserOpen: 'playlist-browser:open',
     sessionNumericSelectionStart: 'session:numeric-selection-start',