Fix periodic tvOS playback stutter from render stalls on mpvQueue

Build 264 stutters for a split second every ~10 seconds. The periodic
playback-stats task fetches ~30 debug properties in one ~190ms block on
mpvQueue, and MPVClient.render() wrapped mpv_render_context_render in
mpvQueue.sync - so the render thread queued behind the fetch and stalled
~190ms, at exactly the stats interval. Build 261 predates the stats
task, which is why it was unaffected.

libmpv's render API is documented thread-safe relative to the client
API, so render-context calls no longer go through mpvQueue:

- New renderContextLock protects renderContext lifecycle vs use;
  render(), renderWithDepth(), renderSoftware(), reportSwap(),
  shouldRenderUpdateFrame() and hasRenderContext use it
- Render-update callbacks dispatch on a dedicated renderEventQueue so
  frame-ready notifications can't queue behind property/command work
- destroyRenderContext takes mpvQueue then the lock, still excluding
  in-flight renders during teardown
- The stats task skips its property fetch on iOS/tvOS unless verbose
  MPV logging is on (macOS keeps it for the render watchdog)
- performRender slow-frame warnings now break down render vs present
  time and report frame-delivery gaps for future triage

Yattee/Services/Player/MPVBackend.swift

@@ -2063,6 +2063,15 @@ extension MPVBackend: MPVClientDelegate {
                 try? await Task.sleep(for: .seconds(10))
                 guard let self, !Task.isCancelled, let client = self.mpvClient else { return }
 
+                #if !os(macOS)
+                // Only macOS needs the property fetch every tick (render
+                // watchdog below). Elsewhere the batched mpv_get_property
+                // calls contend with the playback core and caused a visible
+                // stutter every 10s on tvOS, so skip the fetch entirely
+                // unless verbose logging wants the stats line.
+                guard MPVLogging.verboseEnabled else { continue }
+                #endif
+
                 let props = await client.getDebugPropertiesAsync()
 
                 #if os(macOS)