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>

CHANGELOG.md

@@ -331,6 +331,43 @@ read any of them:
   lock stays busy past the same 5s bound the change is handed to the update
   worker, which applies the latest one as soon as the lock frees, and before
   the plugin's next update() at the latest. The plugin API is unchanged.
+- With a Vegas width budget set (`max_plugin_width_ratio` or a plugin's
+  `vegas_max_width_screens`), a single image over the budget with no gaps
+  between items -- a map, one long headline -- no longer takes a pass of its
+  own showing four blank columns. The cut landed in the middle of the blank
+  margin trimming leaves at the image's edge; margins are no longer cut
+  points, so such an image is cropped to the budget as intended.
+
+### Live Vegas elements (plugin API)
+
+- New plugin hooks for content that can change while it scrolls:
+  `BasePlugin.get_vegas_elements()` returns `VegasElement`s -- named,
+  fixed-width pieces of Vegas content -- instead of pictures;
+  `redraw_vegas_element(key, width, height, at)` redraws one without the
+  plugin lock for content that changes with time; and
+  `notify_vegas_data_changed()` reports data that arrived outside
+  `update()`. New module `src/plugin_system/vegas_elements.py`
+  (`VegasElement`, also re-exported from `base_plugin`). See "Live Vegas
+  elements" in `docs/PLUGIN_API_REFERENCE.md`.
+- The ticker asks a plugin that implements the hook for elements on its
+  background fetch (under the plugin's lock, on a canvas of its own) and
+  records where each one lands in the strip, in absolute columns a trim does
+  not move (`src/vegas_mode/elements.py`). Live elements are never trimmed to
+  their ink: each is padded with `content_padding` black columns either side.
+  Every other path -- the first strip, the render-thread fallback, plugins
+  without the hook -- is unchanged. Swapping redraws into the strip builds
+  on this.
+- `PluginManager.add_update_listener()` / `remove_update_listener()` /
+  `notify_data_changed()`: a listener hears a plugin id the moment its
+  `update()` completes, rather than at the next ~4s Vegas poll.
+- New `display.vegas_scroll` settings: `live_refresh` (default `true`; the
+  kill switch), `live_max_hz`, `live_min_interval`, `live_lead_screens`, and
+  a per-plugin core-owned `vegas_live`. Live elements are off whatever these
+  say under multi-display sync, in swap mode and with `offscreen_prefetch`
+  off.
+- `scripts/check_plugin.py` checks the element contract for any plugin that
+  implements it (`src/plugin_system/testing/vegas.py`), and
+  `test/fixtures/plugins/vegas-live-stub` is a working example.
 
 ### Scrolling