feat(vegas): prefetch_gate on by default, after an A/B/C soak on hdpi

Two runs per arm, about 81,000 frames each, order A B C C B A:

  A  step 1 as is            0.90% late, 20.1 per 10k two+ refreshes late
  B  switch_interval_ms 1    0.78% late, 15.8 per 10k
  C  prefetch_gate           0.60% late,  2.5 per 10k

No freezes in any arm, and the next group was ready at every strip
extension, so parking the prefetch thread (3-6s per 8-minute run) cost
nothing visible. The gate is now on unless vegas_scroll.prefetch_gate is
false; on a stock binding it cannot work and says so at INFO once a run
rather than warning on every install. switch_interval_ms stays off.

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

src/vegas_mode/coordinator.py

@@ -340,12 +340,14 @@ class VegasModeCoordinator:
         if getattr(self.display_manager, 'render_gate', None) is not None:
             return
         releases = render_gate.swap_releases_gil()
+        if releases is None:
+            logger.debug("Vegas: no prefetch gate -- no hardware binding loaded")
+            return
         if not releases:
-            logger.warning(
-                "Vegas: prefetch_gate ignored -- %s",
-                "this rgbmatrix binding keeps the GIL in SwapOnVSync "
-                "(scripts/build_rgbmatrix_nogil.sh)" if releases is False
-                else "no hardware binding loaded")
+            # On by default, so this is every stock install: say so once per
+            # run, not as a warning.
+            logger.info("Vegas: no prefetch gate -- this rgbmatrix binding keeps "
+                        "the GIL in SwapOnVSync (scripts/build_rgbmatrix_nogil.sh)")
             return
         gate = render_gate.RenderGate()
         # Locks the render thread takes too: never park the prefetch holding one.