refactor(main): move core layers into assemblies (#310)

* 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

docs/workflow/testing.md

@@ -47,3 +47,7 @@ good test looks like here.
 - `src/main/main-wiring.test.ts` and `src/renderer/renderer-init-order.test.ts` still
   regex `main.ts` / `renderer.ts` to guard startup ordering. Replace each test with a
   behavioral one as the logic moves out of `main.ts` closures.
+- `src/main/runtime/overlay-geometry-runtime.ts` and
+  `src/main/runtime/visible-overlay-interaction-runtime.ts` value-import Electron, so they
+  cannot load under `bun test` and stay outside `src/main/assemblies/overlay.ts`. Inject
+  `screen`/`dialog` to make them testable and movable.