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/lifecycle.lua

@@ -162,12 +162,15 @@ function M.create(ctx)
 			and not (state.overlay_running and state.auto_play_ready_signal_seen == true)
 	end
 
+	-- A warm playlist advance keeps the already-ready overlay window, and the app only reports
+	-- readiness when content first loads, so a restarted spinner would never clear (#315).
 	local function should_show_overlay_loading_osd()
 		return (
 				resolve_overlay_loading_osd_enabled()
 				or (resolve_auto_start_enabled() and resolve_auto_start_visible_overlay_enabled())
 			)
 			and not state.suppress_ready_overlay_restore
+			and not (state.overlay_running and state.overlay_content_ready_seen == true)
 	end
 
 	local function start_overlay_when_socket_ready(generation, media_identity, same_media_loaded, attempt)
@@ -311,6 +314,7 @@ function M.create(ctx)
 		process.stop_overlay_loading_osd()
 		clear_pending_visible_overlay_hide()
 		state.auto_play_ready_signal_seen = false
+		state.overlay_content_ready_seen = false
 		state.current_media_identity = nil
 		state.current_media_title = nil
 		state.pending_reload_media_identity = nil