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_vegas_live_integration.py

@@ -122,6 +122,64 @@ def test_an_update_moves_the_plugins_epoch_and_new_elements_carry_it(ticker):
     assert metas and all(m.epoch == epoch for m in metas)
 
 
+def _columns(pipeline, record):
+    x = record.abs_x - pipeline._strip_origin
+    return pipeline.scroll_helper.cached_array[:, max(0, x):x + record.width].copy()
+
+
+def _upcoming(pipeline, key_prefix):
+    """A live record of this kind that is not yet behind the viewport."""
+    left = pipeline._strip_origin + int(pipeline.scroll_helper.scroll_position)
+    for record in pipeline.live_records():
+        if record.key.startswith(key_prefix) and record.abs_x + record.width > left + 40:
+            return record
+    return None
+
+
+def test_an_update_changes_cards_already_in_the_strip(ticker):
+    coordinator, plugin, pm = ticker
+    assert coordinator.start()
+    pipeline = coordinator.render_pipeline
+    assert _run_until(coordinator, lambda: _upcoming(pipeline, "card:"))
+    record = _upcoming(pipeline, "card:")
+    before = _columns(pipeline, record)
+
+    plugin.update()                    # new data: every card's bars change
+    pm._note_update_completed(PID)     # what the update worker does after it
+    assert _run_until(
+        coordinator,
+        lambda: not np.array_equal(_columns(pipeline, record), before), seconds=5.0)
+    # The card changed in place: same columns, same width, new pixels.
+    assert pipeline._record_by_seq[record.seq] == record
+    assert pipeline._live_worker is not None and pipeline._live_worker.is_alive()
+
+
+def test_an_animated_element_moves_with_no_update_at_all(ticker):
+    coordinator, _plugin, _pm = ticker
+    assert coordinator.start()
+    pipeline = coordinator.render_pipeline
+    assert _run_until(coordinator, lambda: _upcoming(pipeline, "map"))
+    record = _upcoming(pipeline, "map")
+    before = _columns(pipeline, record)
+    assert _run_until(
+        coordinator,
+        lambda: (pipeline._record_by_seq.get(record.seq) is not None
+                 and not np.array_equal(_columns(pipeline, record), before)),
+        seconds=5.0)
+
+
+def test_stopping_vegas_stops_the_worker(ticker):
+    coordinator, _plugin, _pm = ticker
+    assert coordinator.start()
+    pipeline = coordinator.render_pipeline
+    assert _run_until(coordinator, lambda: pipeline._live_worker is not None)
+    worker = pipeline._live_worker
+    coordinator.stop()
+    worker.join(3)
+    assert not worker.is_alive()
+    assert pipeline._live_worker is None
+
+
 def test_with_live_refresh_off_the_same_run_is_plain_content(dm, tmp_path, ticker):
     coordinator, _plugin, _pm = ticker
     coordinator.vegas_config.live_refresh = False