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

plugin/subminer/state.lua

@@ -14,6 +14,9 @@ function M.new()
 		auto_play_ready_osd_timer = nil,
 		auto_play_ready_signal_seen = false,
 		auto_play_ready_initial_pause_ownership_consumed = false,
+		-- The running overlay has reported its content ready at least once; warm playlist advances
+		-- reuse that window, so they must not restart the loading spinner (#315).
+		overlay_content_ready_seen = false,
 		overlay_loading_osd_active = false,
 		overlay_loading_osd_timer = nil,
 		overlay_loading_osd_deadline = nil,