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>

docs/CONFIG_REFERENCE.md

@@ -127,7 +127,8 @@ Read by `src/vegas_mode/config.py` (`VegasScrollConfig.from_config`). See
 | `min_content_separation` | int, `24` |
 | `min_cut_gap` | int, `6` |
 | `continuous_scroll` | bool, `true` |
-| `smooth_scroll` | bool, `true` |
+| `smooth_scroll` | bool, `true` — move a whole number of pixels per panel refresh, locked to vsync. `scroll_speed` is snapped to the nearest speed the panel can show that way (at 95Hz: 95, 47.5, 31.7 px/s…), measured against the panel's real refresh rate once scrolling starts |
+| `sub_pixel_blend` | bool, `false` — the older smoothing: advance by elapsed time and blend neighbouring pixel columns. Looks anti-aliased in the web preview but shimmers on the panel and is not locked to the refresh. Overrides `smooth_scroll` when on |
 | `extend_threshold_screens` | float, `2.0` |
 | `auto_trim` | bool, `true` |
 | `trim_threshold` | int, `10` |