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/blueprints/api_v3/config.py

@@ -802,7 +802,7 @@ def save_main_config():
                        'vegas_intra_plugin_gap', 'vegas_render_width_pct',
                        'vegas_min_content_separation', 'vegas_min_cut_gap',
                        'vegas_continuous_scroll', 'vegas_extend_threshold_screens',
-                       'vegas_smooth_scroll', 'vegas_overflow_mode']
+                       'vegas_smooth_scroll', 'vegas_overflow_mode', 'vegas_live_refresh']
 
         if any(k in data for k in vegas_fields):
             if 'display' not in current_config:
@@ -822,6 +822,7 @@ def save_main_config():
             _set_checkbox(vegas_config, 'dynamic_duration_enabled', 'vegas_dynamic_duration_enabled')
             _set_checkbox(vegas_config, 'continuous_scroll', 'vegas_continuous_scroll')
             _set_checkbox(vegas_config, 'smooth_scroll', 'vegas_smooth_scroll')
+            _set_checkbox(vegas_config, 'live_refresh', 'vegas_live_refresh')
 
             # max_plugin_width_ratio is the one fractional setting, so it is
             # handled outside the integer loop below.