fix(vegas): keep the sub-pixel frame interval #637 removed as dead code

On main, RenderPipeline._frame_interval was set and never read, so #637
dropped it. Here the frame_interval property reads it for the sub-pixel
path (the crisp path solves its own), so the merge brings it back, with a
comment saying who reads it. #637's new coordinator test drives a
MagicMock pipeline, which this branch's pacing reads frame_interval and
target_fps from; give the mock real ones.

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)