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

docs-site/websocket-texthooker-api.md

@@ -227,7 +227,7 @@ The mpv plugin accepts these script messages:
 script-message subminer-start backend=hyprland socket=/custom/path texthooker=no log-level=debug
 ```
 
-The plugin also registers `subminer-autoplay-ready`, `subminer-autoplay-hold`, `subminer-visible-overlay-shown`, `subminer-visible-overlay-hidden`, `subminer-managed-subtitles-loading`, `subminer-overlay-loading-ready`, and `subminer-reload-session-bindings`. The SubMiner app sends these to keep the plugin in sync, so do not send them from your own scripts.
+The plugin also registers `subminer-autoplay-ready`, `subminer-autoplay-hold`, `subminer-visible-overlay-shown`, `subminer-visible-overlay-hidden`, `subminer-managed-subtitles-loading`, `subminer-overlay-loading-ready`, `subminer-overlay-loading-dismissed`, and `subminer-reload-session-bindings`. The SubMiner app sends these to keep the plugin in sync, so do not send them from your own scripts.
 
 While the app is connected to mpv, it also handles two AniSkip messages over the mpv IPC socket: `subminer-skip-intro` skips the intro, and `subminer-aniskip-refresh` reloads intro data, for example after your script changes title or episode metadata.