feat(vegas): live elements update in place while they scroll (#697)

* perf(timing): say which render-thread work a late frame followed

The soak already says how often a moving frame reached the panel late, but
not what the render thread was doing just before it. Vegas does two kinds of
work there between frames -- building its strip (compose, extend) and, with
live elements, patching changed pixels into it -- and deciding whether either
is affordable needs their own numbers.

- FrameTimingRecorder.note_op(kind, nbytes) tags the next presented frame.
  Totals gain op_frames, late_op_frames, op_freezes and op_bytes per kind;
  aggregate() still takes frames without ops. The file schema is unchanged.
- Vegas tags compose and every strip extension (with the bytes it copied).
- frame_soak prints an "after work" table: frames, late %, freezes and MB
  moved per kind, only when something tagged its work.
- render_bench gains --strip-screens (Vegas-sized strips), --patch-bytes /
  --patch-every / --patch-where (in-place column writes, as a live element
  update does) and --extend-every-screens / --extend-width (append + trim on
  a fixed cadence that holds the strip's width).

No runtime behaviour changes: this is the measurement gate for live Vegas
elements.

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

* docs(changelog): note the frame-op attribution and bench modes

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

* perf(scroll): build the strip's PIL image only when something reads it

Every Vegas strip extension rebuilt ScrollHelper.cached_image from
cached_array in full, twice (append, then trim), on the render thread:
Image.fromarray is 1.7ms for an 8,000px strip and 3.8ms for 20,000px on a
Pi 4 (measured on ledpi), about two thirds of an extension's render-thread
cost. Nothing on the frame path reads the image's pixels; every frame is cut
from the array.

cached_image is now a property. append_content and drop_scrolled_prefix
defer it; the first read builds it from the array it started with and keeps
it only if the strip has not changed meanwhile, so a sync push racing an
extension cannot leave a stale image cached. Assigning cached_image stores
exactly what was assigned, as before. has_strip() says whether there is a
strip without building its image; the helper's frame path, Vegas and the
adapter's scroll-cache invalidation use it. The strip is also no longer held
in memory twice.

In Vegas the image is now built only by a multi-display sync push.

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

* feat(vegas): live elements -- a plugin API for content that changes while it scrolls

Vegas bakes each plugin's pictures into one strip, so a card already on its
way across the panel keeps what it showed when it was drawn. This adds the
API and bookkeeping for content that can be updated in place; the worker
that redraws and swaps it follows separately. No shipped plugin implements
the hook yet, so nothing changes for users.

Plugin API (core 3.8.0), all no-ops by default:
- BasePlugin.get_vegas_elements() -> [VegasElement(key, image, version,
  live, refresh_hz)]: named, fixed-width pieces of Vegas content.
- BasePlugin.redraw_vegas_element(key, width, height, at): a lock-free
  redraw for content that changes with time.
- BasePlugin.notify_vegas_data_changed(): data that lands outside update().
- src/plugin_system/vegas_elements.py (VegasElement, re-exported from
  base_plugin).

Core:
- PluginAdapter asks a plugin that implements the hook for elements on the
  background fetch only (under its lock, on its own canvas); every other
  path keeps get_vegas_content(). Live elements are pinned (padded with
  content_padding, never trimmed), tagged with their key, digest and data
  epoch in Image.info so the existing cache and group plumbing carry them
  unchanged, and untagged if a width budget crops them.
- RenderPipeline records where each live element lands (ElementRecord), in
  absolute strip columns a trim does not move; the block-start arithmetic
  is shared with the STATIC markers.
- PluginManager update listeners (add/remove_update_listener,
  notify_data_changed): told the moment update() completes, not at the
  next ~4s Vegas poll. The coordinator uses one to move each plugin's data
  epoch on.
- vegas_scroll.live_refresh (kill switch), live_max_hz, live_min_interval,
  live_lead_screens; per-plugin core-owned vegas_live. Live elements are
  off under multi-display sync, in swap mode and with offscreen_prefetch off.
- scripts/check_plugin.py checks the element contract
  (src/plugin_system/testing/vegas.py); test/fixtures/plugins/vegas-live-stub
  is a working example.

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

* 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>

---------

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