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>

src/common/scroll_helper.py

@@ -780,6 +780,43 @@ class ScrollHelper:
         )
         return cut
 
+    def patch_columns(self, x: int, pixels: np.ndarray) -> int:
+        """Overwrite the strip's columns from ``x`` with ``pixels``, in place.
+
+        What a live Vegas element update is (src/vegas_mode/elements.py): the
+        strip keeps its width, the scroll keeps its position, and only these
+        columns change. Call it between frames on the thread that draws them;
+        every frame copies its slice out of the strip (get_visible_portion),
+        so no frame already handed on can see half a patch.
+
+        Clipped to the strip at both ends. Refused (0) for an array this
+        helper may not write -- the multi-display follower adopts a read-only
+        one -- or for pixels of another height. The PIL image is deferred, so
+        a later read of cached_image shows the patch.
+
+        Args:
+            x: Strip column of the first column of ``pixels``
+            pixels: uint8 array (height, width, 3)
+
+        Returns:
+            Bytes written.
+        """
+        strip = self.cached_array
+        if strip is None or not strip.flags.writeable:
+            return 0
+        if pixels.ndim != 3 or pixels.shape[0] != strip.shape[0] \
+                or pixels.shape[2] != strip.shape[2]:
+            return 0
+        width = pixels.shape[1]
+        lo, hi = max(0, int(x)), min(strip.shape[1], int(x) + width)
+        if hi <= lo:
+            return 0
+        strip[:, lo:hi] = pixels[:, lo - int(x):hi - int(x)]
+        if self.__dict__.get('_cached_image') is not None \
+                or self.__dict__.get('_image_source') is not None:
+            self._defer_image()
+        return (hi - lo) * strip.shape[0] * strip.shape[2]
+
     def remaining_unscrolled(self) -> int:
         """Columns of strip still to the right of the viewport."""
         if not self.has_strip():