fix(apple/stage2): disable layer vsync wait to kill fullscreen stutter

The experimental stage-2 presenter (CAMetalLayer + display link) stuttered badly
in fullscreen but ran fine windowed. render() runs on the display-link / MAIN
thread and calls layer.nextDrawable(), which blocks that thread until a drawable
frees. With the layer's own displaySyncEnabled left on (default), present also
waits for the hardware vsync, so the block serializes the main thread to the
display — windowed, the WindowServer's looser compositing hides it; fullscreen's
tighter, more-direct path exposes it as judder. (Apple dev-forum guidance:
displaySync off measurably reduces nextDrawable() blocking.)

- displaySyncEnabled = false (macOS-only): the display link is already the per-
  vsync pacing source, so the layer's redundant vsync wait only adds the stall.
- maximumDrawableCount = 3 (explicit): more in-flight headroom before
  nextDrawable() has to block on the main thread.

Swift-only (no core/ABI change → no xcframework rebuild). Validated: swift build;
swift test (39 passed, 0 failures).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/apple/Sources/PunktfunkKit/MetalVideoPresenter.swift

@@ -80,6 +80,19 @@ public final class MetalVideoPresenter {
         layer.pixelFormat = .bgra8Unorm
         layer.framebufferOnly = true
         layer.isOpaque = true
+        // Triple-buffer: more in-flight drawables before `nextDrawable()` (called on the
+        // display-link / MAIN thread) has to block waiting for one to free.
+        layer.maximumDrawableCount = 3
+        #if os(macOS)
+        // The display link already paces exactly one present per vsync. Leaving the layer's
+        // own vsync wait on means `commandBuffer.present` ALSO blocks for the hardware vsync,
+        // so `nextDrawable()` stalls the MAIN thread until a drawable frees — windowed, the
+        // WindowServer's looser compositing hides it; FULLSCREEN's tighter, more-direct path
+        // serializes the main thread to the display and the stall surfaces as bad judder.
+        // Disabling the layer-level sync lets present return promptly (the display link is the
+        // pacing source), which is what fixes the fullscreen stutter. macOS-only property.
+        layer.displaySyncEnabled = false
+        #endif
         self.layer = layer
     }