fix(vegas): smooth Vegas scroll pacing -- whole pixels per refresh, measured refresh, off-thread preview writes (#628)

Vegas scrolls a whole number of pixels per panel refresh, locked to SwapOnVSync, against the refresh the panel really holds (measured from swap gaps), instead of blending sub-pixel positions against the refresh cap. The web preview PNG is encoded off the render thread while scrolling, with writes ordered and retried. On hdpi, late frames fell from 6.3% to 0.7%. See docs/SCROLL_PERFORMANCE.md.

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

test/test_vegas_coordinator_iteration.py

@@ -21,6 +21,9 @@ def _coordinator(plugins):
     coord.vegas_config = VegasModeConfig.from_config({'display': {'vegas_scroll': {
         'enabled': True, 'max_cycle_duration': 0}}})
     coord.render_pipeline = MagicMock()
+    # The loop paces itself from these (#628); a MagicMock can't be compared.
+    coord.render_pipeline.frame_interval = 0.0
+    coord.render_pipeline.target_fps = 90
     coord.stream_manager = MagicMock()
     coord.display_manager = MagicMock()
     coord.plugin_manager = SimpleNamespace(plugins=plugins, get_plugin=plugins.get)