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>

src/display_manager.py

@@ -915,8 +915,7 @@ class DisplayManager:
         ``display.dirty_tracking: false`` if a redraw issue is ever suspected.
 
         Serialized via ``_update_lock``: plugins can call this directly from
-        background threads (e.g. sports base classes push an immediate
-        "live" refresh from inside update()), so without a lock two callers
+        background threads of their own, so without a lock two callers
         could both pass the digest check before either writes it back,
         double-pushing a frame, or interleave the offscreen/current canvas
         swap below. The lock is scoped to this method, so callers never