feat: wire up subtitle prefetch service to MPV events

Initializes prefetch on external subtitle track activation, detects
seeks via time-pos delta threshold, pauses prefetch during live
subtitle processing, and restarts on cache invalidation.

- Extract loadSubtitleSourceText into reusable function
- Add prefetch service state and initSubtitlePrefetch helper
- Thread onTimePosUpdate through event actions/bindings/main-deps
- Pause prefetch on subtitle change, resume on emit
- Restart prefetch after tokenization cache invalidation
- Query track-list on media path change to find external subs

src/main/runtime/mpv-main-event-actions.ts

@@ -90,11 +90,13 @@ export function createHandleMpvTimePosChangeHandler(deps: {
   recordPlaybackPosition: (time: number) => void;
   reportJellyfinRemoteProgress: (forceImmediate: boolean) => void;
   refreshDiscordPresence: () => void;
+  onTimePosUpdate?: (time: number) => void;
 }) {
   return ({ time }: { time: number }): void => {
     deps.recordPlaybackPosition(time);
     deps.reportJellyfinRemoteProgress(false);
     deps.refreshDiscordPresence();
+    deps.onTimePosUpdate?.(time);
   };
 }
 

src/main/runtime/mpv-main-event-bindings.ts

@@ -59,6 +59,7 @@ export function createBindMpvMainEventHandlersHandler(deps: {
   recordPlaybackPosition: (time: number) => void;
   recordMediaDuration: (durationSec: number) => void;
   reportJellyfinRemoteProgress: (forceImmediate: boolean) => void;
+  onTimePosUpdate?: (time: number) => void;
   recordPauseState: (paused: boolean) => void;
 
   updateSubtitleRenderMetrics: (patch: Record<string, unknown>) => void;
@@ -124,6 +125,7 @@ export function createBindMpvMainEventHandlersHandler(deps: {
       reportJellyfinRemoteProgress: (forceImmediate) =>
         deps.reportJellyfinRemoteProgress(forceImmediate),
       refreshDiscordPresence: () => deps.refreshDiscordPresence(),
+      onTimePosUpdate: (time) => deps.onTimePosUpdate?.(time),
     });
     const handleMpvPauseChange = createHandleMpvPauseChangeHandler({
       recordPauseState: (paused) => deps.recordPauseState(paused),

src/main/runtime/mpv-main-event-main-deps.ts

@@ -47,6 +47,7 @@ export function createBuildBindMpvMainEventHandlersMainDepsHandler(deps: {
   updateCurrentMediaTitle: (title: string) => void;
   resetAnilistMediaGuessState: () => void;
   reportJellyfinRemoteProgress: (forceImmediate: boolean) => void;
+  onTimePosUpdate?: (time: number) => void;
   updateSubtitleRenderMetrics: (patch: Record<string, unknown>) => void;
   refreshDiscordPresence: () => void;
   ensureImmersionTrackerInitialized: () => void;
@@ -134,6 +135,7 @@ export function createBuildBindMpvMainEventHandlersMainDepsHandler(deps: {
     },
     reportJellyfinRemoteProgress: (forceImmediate: boolean) =>
       deps.reportJellyfinRemoteProgress(forceImmediate),
+    onTimePosUpdate: deps.onTimePosUpdate ? (time: number) => deps.onTimePosUpdate!(time) : undefined,
     recordPauseState: (paused: boolean) => {
       deps.appState.playbackPaused = paused;
       deps.ensureImmersionTrackerInitialized();