a plugin API for content that changes while it scrolls (#696)

* 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>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

scripts/check_plugin.py

@@ -72,6 +72,7 @@ from src.plugin_system.testing.harness import (  # noqa: E402
 from src.plugin_system.testing.sizes import (  # noqa: E402
     parse_size_token, resolve_test_sizes, safe_mode_filename, size_label,
 )
+from src.plugin_system.testing.vegas import check_plugin_vegas_elements  # noqa: E402
 
 logger = get_logger("[Check Plugin]")
 
@@ -193,6 +194,24 @@ def check_one(plugin_id: str, search_dirs: List[str], sizes, mock_data: Dict,
 
         all_run_results.extend(results)
 
+    # Live Vegas elements, for a plugin that has them: checked once, at the
+    # first size, with the base config.
+    width, height = effective_sizes[0]
+    try:
+        vegas = check_plugin_vegas_elements(
+            plugin_id, plugin_dir, full_config, effective_mock_data, width, height,
+            run_update=effective_run_update)
+    except Exception as exc:  # noqa: BLE001 - one plugin must not end an --all run
+        all_run_results.append(RenderResult(
+            plugin_id, width, height, "vegas elements",
+            error=f"the element check itself failed: {exc!r}"))
+        return all_run_results
+    if vegas.implemented:
+        all_run_results.append(RenderResult(
+            plugin_id, width, height, "vegas elements",
+            error="; ".join(vegas.errors) or None,
+            notes=[f"{vegas.elements} element(s), {vegas.live} live"] + vegas.warnings))
+
     return all_run_results
 
 
@@ -236,6 +255,8 @@ def print_report(all_results: Dict[str, List[RenderResult]]) -> bool:
                               f" controller skips the mode")
                 else:
                     status, detail = "FAIL", ""
+            if r.notes:
+                detail += f" ({'; '.join(r.notes)})"
             print(f"  [{status}] {r.size_label:>7}  {r.mode}{detail}")
     print()
     return everything_ok