episode_navigation.dart fused two responsibilities: the episode
next/previous vertical and the screen's generic in-place media
transition engine (_switchPlaybackSource, _performPlaybackSourceSwitch,
_selectSourceSubtitleLocally, _reloadMediaInPlace,
_reapplyScopedPlayerPrefsForItemChange), which lifecycle restore,
spurious-EOF recovery, Watch Together, and the companion remote also
drive. Move the engine and deferTranscodeSubtitleSelection verbatim
into playback_reload.dart so the episode part contains only the
episode feature. No code changes beyond the file split.
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