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/mpv-plugin.md

@@ -124,7 +124,7 @@ script-message subminer-start backend=hyprland socket=/custom/path texthooker=no
 
 `log-level` sets SubMiner's log verbosity. Do not use `--debug` for this; it turns on the app's dev mode.
 
-The plugin also handles messages the SubMiner app sends it (`subminer-autoplay-ready`, `subminer-autoplay-hold`, `subminer-visible-overlay-shown`, `subminer-visible-overlay-hidden`, `subminer-managed-subtitles-loading`, `subminer-overlay-loading-ready`, `subminer-reload-session-bindings`). You do not need to send these yourself. The AniSkip messages are listed on the [AniSkip page](/aniskip-integration#triggering-from-mpv).
+The plugin also handles messages the SubMiner app sends it (`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`, `subminer-reload-session-bindings`). You do not need to send these yourself. The AniSkip messages are listed on the [AniSkip page](/aniskip-integration#triggering-from-mpv).
 
 ## Auto-start behavior