fix(plugin): keep loading OSD off on warm playlist advance (#317)

* fix(plugin): keep loading OSD off on warm playlist advance

- Track overlay_content_ready_seen when the app sends subminer-overlay-loading-ready
- Skip restarting the "Overlay loading" spinner for an overlay that already reported ready, so it no longer times out into "Overlay did not become ready" (#315)
- Reset the flag on stop, restart, start failure and shutdown
- Add start-gate tests for warm and not-yet-ready playlist advances

* fix(plugin): only report overlay ready once content has loaded

Hide paths dismissed the loading OSD with subminer-overlay-loading-ready,
which the plugin now remembers to skip the spinner on later playlist files.
A hide before the overlay had content would wrongly mark it ready. The app
now sends ready only when the overlay window content is ready, and a new
stop-only subminer-overlay-loading-dismissed message otherwise.

* fix(plugin): resync overlay readiness on dismissal and mpv connect

A dismissal now clears the plugin's readiness flag, since the app sends it
only while its overlay window has no content (e.g. a replaced window).
On mpv (re)connect the app resends subminer-overlay-loading-ready when the
overlay already has content, so a ready signal lost while mpv was away, or
sent before this mpv's plugin existed, still reaches the plugin.

* refactor(main): pass overlay readiness handler directly

src/main/assemblies/playback.ts

@@ -205,6 +205,7 @@ export interface PlaybackAssemblyDeps {
   ) => SubtitlePrefetchService;
   requestAppQuit: () => void;
   maybeStartOverlayLoadingOsd: OverlayNotificationsRuntime['maybeStartOverlayLoadingOsd'];
+  reportOverlayContentReadyToPlugin: OverlayNotificationsRuntime['reportOverlayContentReadyToPlugin'];
   flushQueuedMpvOsdNotifications: OverlayNotificationsRuntime['flushQueuedMpvOsdNotifications'];
   refreshMpvSessionBindings: () => Promise<void>;
   ensureImmersionTrackerStarted: () => void;
@@ -589,6 +590,7 @@ export function createPlaybackAssembly(deps: PlaybackAssemblyDeps) {
       },
       onMpvConnected: () => {
         deps.maybeStartOverlayLoadingOsd();
+        deps.reportOverlayContentReadyToPlugin();
         deps.flushQueuedMpvOsdNotifications();
         secondarySubtitleTrackController.scheduleRefresh(0);
         void deps.refreshMpvSessionBindings();