fix(subtitle-prefetch): reuse startup runs and prefetch overlap lines

- Warm combined text for overlapping subtitle cues.
- Preserve valid in-flight tokenization across startup restarts while rejecting invalidated cache results.

changes/prefetch-startup-restarts.md

@@ -0,0 +1,5 @@
+type: fixed
+area: annotations
+
+- Subtitle lines shown while two cues overlap (common in Jimaku and Jellyfin SRTs, and usually the long lines) are now prefetched, so their annotations appear immediately instead of after a delay.
+- Subtitle prefetching no longer restarts each time the same subtitle file is re-selected during startup, and a line tokenized before a restart or seek is kept instead of discarded.