feat(vegas): live elements update in place while they scroll

One background worker (src/vegas_mode/live_worker.py) redraws a plugin's
live elements when its data epoch moves on (update listener) or on their
refresh_hz, nearest the screen first, and hands changed pixels lock-free to
the render thread, which copies them into the strip between frames
(RenderPipeline.apply_live_patches, ScrollHelper.patch_columns): at most
four patches or two screens of bytes a frame, no drawing or locks there.
The worker takes over group prefetch once a live element is placed, runs
inside the render gate, and is supervised. Update tick 1s while live
elements exist. Web UI switch for live_refresh. OFFSCREEN_RENDERING.md
describes what was built and why SegmentStrip was not needed.

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

web_interface/templates/v3/partials/display.html

@@ -568,6 +568,17 @@
                         </label>
                     </div>
 
+                    <div class="form-group mb-4" id="setting-display-vegas_live_refresh" data-setting-key="display.vegas_scroll.live_refresh">
+                        <label class="flex items-center">
+                            <input type="checkbox"
+                                   id="vegas_live_refresh"
+                                   name="vegas_live_refresh"
+                                   {% if main_config.display.get('vegas_scroll', {}).get('live_refresh', True) %}checked{% endif %}
+                                   class="form-checkbox">
+                            <span class="ml-2 text-sm text-gray-700">Update live content while it scrolls{{ ui.help_tip('For plugins that support it (scores, the flight map), change what is already scrolling when their data changes: a score updates on a card already crossing the screen, and aircraft move on the map. Default: on.\nWith this off every card shows what it was when it was drawn, which can be a minute or more old by the time it scrolls past.\nAlways off with multi-display sync or with Continuous Scroll off.', 'Live Updates') }}</span>
+                        </label>
+                    </div>
+
                     <div class="grid grid-cols-1 md:grid-cols-2 gap-4 mb-4">
                         <div class="form-group" id="setting-display-vegas_extend_threshold_screens" data-setting-key="display.vegas_scroll.extend_threshold_screens">
                             <label for="vegas_extend_threshold_screens" class="block text-sm font-medium text-gray-700">Extend When (screens left){{ ui.help_tip('How much unscrolled content triggers loading the next group, measured in screen widths (1.0–10.0).\nDefault: 2. Higher loads earlier and leaves more slack, at the cost of holding more content in memory. Only applies when Continuous Scroll is on.', 'Extend Threshold') }}</label>