fix(vegas): lock the scroll to the panel refresh; encode the preview off the render thread

Vegas advanced by elapsed time, blended neighbouring columns every frame,
and paced itself with a sleep to target_fps. On hdpi (4x128x64 on one
chain, a 120Hz cap the chain cannot reach, ~95-100Hz real) that ran at
73fps with target 90 and ~89fps with target 125: the sleep drifted
against the refresh and missed a vsync every few frames, and the blend
read as shimmer on the panel (and as "anti-aliased" text in the preview).

smooth_scroll now means the crisp pacing the plugin tickers already use:
a whole number of pixels per presented frame, each held for frame_hold
refreshes, with SwapOnVSync as the clock. The speed is solved against the
panel's measured refresh, timed from our own swaps once scrolling starts,
because the configured limit is only a cap -- at "120Hz" 90px/s solves to
3px every 4 refreshes, at the real ~97Hz to 1px every refresh. The old
blend stays available as sub_pixel_blend (default off).

With the web preview open, the render thread also PNG-encoded the whole
512x64 frame five times a second, 12-14ms each -- longer than a refresh.
Mid-scroll that encode now runs on a single-slot writer thread (Pillow
releases the GIL while compressing); static frames still write inline.

Measured on hdpi, 3-minute soak with the preview open: 3 of 17,280
frames held an extra refresh (0.02%), down from ~6-20%.

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

test/test_display_pending_changes.py

@@ -140,6 +140,9 @@ def vegas_coordinator(controller):
         'enabled': True, 'max_cycle_duration': VEGAS_ITERATION_SECONDS}}})
     assert coord.vegas_config.continuous_scroll
     coord.render_pipeline = MagicMock()
+    # Real numbers: the loop sleeps and reports against these.
+    coord.render_pipeline.frame_interval = coord.vegas_config.get_frame_interval()
+    coord.render_pipeline.target_fps = float(coord.vegas_config.target_fps)
     coord.stream_manager = MagicMock()
     coord.display_manager = controller.display_manager
     coord.stats = {'cycles_completed': 0, 'interruptions': 0}