experiment(vegas): vegas_scroll.prefetch_gate runs the prefetch only while the render thread waits on vsync

The render thread spends most of each refresh in SwapOnVSync with the GIL
released, then needs it back the moment the swap returns. With plugin
rendering on the prefetch thread, it often has to wait for it -- behind
bytecode for up to the switch interval, behind a GIL-holding C call for as
long as that takes -- and hdpi's late frames of 2-5 refreshes went up.

src/common/render_gate.py opens a window around each swap, up to just
before the refresh the swap will return on, and a profile hook on the
prefetch thread parks it outside that window. It is never parked holding a
lock the render thread also takes (the Vegas buffer, cache and state
locks, logging, threading, importlib, the cache), never when no frame has
been swapped for 50ms, and never for more than 50ms at a time.

Off by default and ignored on a binding that keeps the GIL in
SwapOnVSync, where the window would never let the prefetch run.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/vegas_mode/render_pipeline.py

@@ -10,6 +10,7 @@ import os
 import time
 import threading
 from collections import deque
+from contextlib import nullcontext
 from typing import Optional, List, Any, Dict, Deque, TYPE_CHECKING
 from PIL import Image
 
@@ -392,8 +393,12 @@ class RenderPipeline:
                     os.nice(10)
                 except (OSError, AttributeError):
                     pass
+                # With vegas_scroll.prefetch_gate on, run only while the render
+                # thread waits on vsync; see src/common/render_gate.py.
+                gate = getattr(self.display_manager, 'render_gate', None)
                 try:
-                    group = self.stream_manager.take_next_group(offscreen_only=True)
+                    with gate.yielding() if gate is not None else nullcontext():
+                        group = self.stream_manager.take_next_group(offscreen_only=True)
                 except Exception:
                     logger.exception("Background prefetch failed")
                     group = []