feat(vegas): render every plugin's ticker content off the render thread

The plugin-facing canvas (DisplayManager.image, draw, matrix) was one
shared object, so any plugin whose Vegas content needed it -- display
capture, scroll-content generation, narrowed rendering -- was deferred to
the render thread and fetched there one at a time. On hdpi that is most
plugins, and each fetch stalled the scroll: news ~320ms, hockey ~660ms,
in bursts whenever the strip extended.

DisplayManager.offscreen() gives the calling thread a canvas of its own.
image, draw and matrix are now properties that resolve to the thread's
surface while it is inside the block and to the shared canvas otherwise,
so the ~100 existing uses become thread-correct unchanged. Inside,
update_display(), the hardware half of clear(), and set_scrolling_state()/
set_frame_hold() are inert, so a plugin drawn for Vegas can neither reach
the panel nor re-pace the live scroll. render_size() is rebuilt on it.
capture_mode() now restores the previous state instead of clearing it,
so it cannot end suppression inside an offscreen block.

The adapter draws every path on its own canvas (_isolated_canvas) and
drops the copy-and-restore of the shared image, which from a background
thread would have written a stale frame back over the render loop's.
Background fetches take the plugin's update/display lock, waiting up to
2s for a running update() and skipping the plugin that round otherwise;
Vegas never took that lock, so render-thread captures already raced
update(). A background fetch that comes back empty is no longer queued
for the render thread.

vegas_scroll.offscreen_prefetch (default true) restores the old deferred
path when false. See docs/OFFSCREEN_RENDERING.md.

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

src/vegas_mode/render_pipeline.py

@@ -520,9 +520,12 @@ class RenderPipeline:
             grouped = [(pid, imgs) for pid, imgs in grouped if imgs]
 
             if not grouped:
-                # Everything in this group is queued; the queue will extend the
-                # strip as it drains, so this is not a failure.
-                logger.info("Whole group deferred; strip will extend as it drains")
+                if deferred:
+                    # Everything in this group is queued; the queue will extend
+                    # the strip as it drains, so this is not a failure.
+                    logger.info("Whole group deferred; strip will extend as it drains")
+                else:
+                    logger.info("Nothing to show in this group; fetching the next")
                 self.start_prefetch()
                 return bool(deferred)