* refactor(main): move first-run setup into an assembly
* refactor(main): move playback and subtitle pipeline state into an assembly
Moves the mpv runtime composer wiring together with the subtitle processing, prefetch, sidebar playback-state, seek-intent, secondary-subtitle and autoplay-priming state it mutates into createPlaybackAssembly. Replaces the main-wiring regex tests for the moved code with behaviour tests that drive real mpv events through a fake client.
* refactor(main): move overlay windows and visibility into an assembly
* refactor(main): read Electron screen lazily in the overlay assembly deps
* docs: describe overlay, playback, and first-run setup assemblies
* test(playback): clear scheduled prefetch refresh in warm-tokenization tests
