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>

web_interface/templates/v3/partials/display.html

@@ -523,7 +523,7 @@
 
                 <div class="grid grid-cols-1 md:grid-cols-2 gap-4">
                     <div class="form-group" id="setting-display-vegas_target_fps" data-setting-key="display.vegas_scroll.target_fps">
-                        <label for="vegas_target_fps" class="block text-sm font-medium text-gray-700">Target FPS{{ ui.help_tip('Frames per second the Vegas ticker aims to render.\nHigher = smoother scrolling but more CPU. Default: 125 (smoothest). Drop to 60/90 if the Pi runs hot.', 'Target FPS') }}</label>
+                        <label for="vegas_target_fps" class="block text-sm font-medium text-gray-700">Target FPS{{ ui.help_tip('Only used when Smooth motion is off; with it on, the panel refresh sets the frame rate.\nFrames per second the Vegas ticker aims to render. Default: 125.', 'Target FPS') }}</label>
                         <select id="vegas_target_fps" name="vegas_target_fps" class="form-control">
                             <option value="60" {% if main_config.display.get('vegas_scroll', {}).get('target_fps', 125) == 60 %}selected{% endif %}>60 FPS (Lower CPU)</option>
                             <option value="90" {% if main_config.display.get('vegas_scroll', {}).get('target_fps', 125) == 90 %}selected{% endif %}>90 FPS (Balanced)</option>
@@ -564,7 +564,7 @@
                                    name="vegas_smooth_scroll"
                                    {% if main_config.display.get('vegas_scroll', {}).get('smooth_scroll', True) %}checked{% endif %}
                                    class="form-checkbox">
-                            <span class="ml-2 text-sm text-gray-700">Smooth sub-pixel motion{{ ui.help_tip('Blend between neighbouring pixel positions so the ticker moves once per rendered frame instead of once per pixel. Default: on.\nWithout it, motion happens only as often as the scroll speed in pixels per second — at 50 px/s that is 50 steps a second however fast the display renders, which reads as a slight judder. The trade is that text softens very slightly horizontally, since each frame blends two positions. Turn it off if you prefer maximum crispness.', 'Smooth Scrolling') }}</span>
+                            <span class="ml-2 text-sm text-gray-700">Smooth motion{{ ui.help_tip('Move the ticker a whole pixel at a time in step with the panel refresh, so every frame moves the same distance and text stays sharp. Default: on.\nThe scroll speed snaps to the nearest speed the panel can show this way: on a 100Hz panel, 100 px/s is one pixel every refresh and 50 px/s one pixel every second refresh. The real refresh rate is measured once scrolling starts, since a long chain often cannot reach its refresh cap.\nWith this off the ticker moves by elapsed time instead, which drifts against the refresh and judders.', 'Smooth Scrolling') }}</span>
                         </label>
                     </div>