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/types/anki.ts

@@ -82,6 +82,11 @@ export interface MediaTimingReviewPreviewRequest {
   endTime: number;
 }
 
+export interface MediaTimingReviewPreviewPosition {
+  reviewId: string;
+  time: number;
+}
+
 export interface MediaTimingReviewWaveformRequest {
   reviewId: string;
   startTime: number;

src/types/runtime.ts

@@ -6,6 +6,7 @@ import type {
   KikuMergePreviewResponse,
   MediaTimingReviewActionResult,
   MediaTimingReviewOpenPayload,
+  MediaTimingReviewPreviewPosition,
   MediaTimingReviewPreviewRequest,
   MediaTimingReviewResolveRequest,
   MediaTimingReviewFrameRequest,
@@ -556,6 +557,9 @@ export interface ElectronAPI {
   onOpenYoutubeTrackPicker: (callback: (payload: YoutubePickerOpenPayload) => void) => void;
   onOpenMediaTimingReview: (callback: (payload: MediaTimingReviewOpenPayload) => void) => void;
   onMediaTimingReviewPreviewEnded: (callback: (reviewId: string) => void) => void;
+  onMediaTimingReviewPreviewPosition: (
+    callback: (position: MediaTimingReviewPreviewPosition) => void,
+  ) => void;
   previewMediaTimingReview: (
     request: MediaTimingReviewPreviewRequest,
   ) => Promise<MediaTimingReviewActionResult>;