fix(web): forward cues that already exist when a text track is added

[tvanlaerhoven]

Summary

WebEventForwarder.onAddTextTrack attached the track-level addcue listener only when addtrack fired, so any cue already present on the track at that moment produced no player-level TEXT_TRACK/addcue event. On iOS Safari the web SDK populates daterange cues before firing addtrack, so consumers listening on PlayerEventType.TEXT_TRACK never received them (the cues were only visible via event.track.cues on the TEXT_TRACK_LIST event).

Fix: after dispatching ADD_TRACK, replay the cues already on the track as ADD_CUE player events. Their uids are remembered so a native addcue arriving later for one of those same cues is not forwarded twice; a uid is dropped from that set on removecue, so a genuine remove/re-add still forwards. The set only ever holds cues that existed at addtrack time — cues arriving afterwards are forwarded with no bookkeeping.

   dispatchEvent(ADD_TRACK, fromNativeTextTrack(track));
+  track.cues?.forEach((cue) => dispatchEvent(ADD_CUE, track.uid, fromNativeCue(cue)));

Second, unrelated-but-adjacent fix: track listener removal never worked. onRemoveTextTrack / onRemoveMediaTrack called removeEventListener with a freshly created closure (this.onAddTextTrackCue(track)), a different function identity than the one registered — so cue and activequalitychanged listeners stayed attached for the life of the player, leaking and dispatching events for removed tracks. The listeners are now kept per track uid and the exact registered reference is removed; removeEventListeners() also detaches tracks that were never explicitly removed.

Note the same attach-on-ADD_TRACK pattern exists in the iOS (THEOplayerRCTTextTrackEventHandler) and Android (PlayerEventEmitter) handlers; those are left alone since the native SDKs fire addcue after addtrack.

Verification

No JS test framework exists in this repo, so this was verified with a standalone script driving the compiled forwarder against a fake player/track emitter, covering: cues present at addtrack are forwarded after ADD_TRACK; a duplicate native addcue for a replayed cue is suppressed; remove-then-re-add of the same cue is forwarded again; no cue events after removetrack. npm run lint, npm run prettier and tsc -p tsconfig.build.json pass.

Link to Devin session: https://dolby.devinenterprise.com/sessions/fc9bf7b3e83e4f2bafb75a3d70edcb3c
Requested by: @tvanlaerhoven

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