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>

test/test_api_v3_partial_main_save.py

@@ -119,6 +119,21 @@ class TestJsonPartialSaves:
                     'continuous_scroll', 'smooth_scroll'):
             assert vegas[key] is True, key
 
+    def test_vegas_speed_only_keeps_live_updates_as_stored(self, api_v3_client, saved):
+        resp = _post_json(api_v3_client, {'vegas_scroll_speed': 70})
+        assert resp.status_code == 200, resp.get_json()
+        assert 'live_refresh' not in saved['config']['display']['vegas_scroll']
+
+    def test_live_updates_can_be_switched_off_and_on(self, api_v3_client, saved):
+        resp = _post_json(api_v3_client, {'vegas_live_refresh': False})
+        assert resp.status_code == 200, resp.get_json()
+        assert saved['config']['display']['vegas_scroll']['live_refresh'] is False
+        resp = _post_json(api_v3_client, {'__form_section': 'display',
+                                          'vegas_scroll_speed': '50',
+                                          'vegas_live_refresh': 'on'})
+        assert resp.status_code == 200, resp.get_json()
+        assert saved['config']['display']['vegas_scroll']['live_refresh'] is True
+
     def test_double_sided_axis_only_keeps_enabled(self, api_v3_client, saved):
         resp = _post_json(api_v3_client, {'double_sided_axis': 'horizontal'})
         assert resp.status_code == 200, resp.get_json()