feat(perf): time every presented frame, and a soak script to judge a rig

Each scroller already logs its own stats line, but in different formats,
per source, and Vegas logs a healthy window only at DEBUG. None of it
answers the question a release has to answer on each rig: over a long
run, how often did a moving frame reach the panel late?

Every frame reaches the panel through DisplayManager.update_display, so
it is timed there once, whoever drew it: the blit (SetImage), the vsync
wait, and the interval since the previous frame. The render thread only
appends a tuple. A worker thread aggregates cumulative counters and
histograms and rewrites /dev/shm/ledmatrix_frame_stats.json every 10s
(RAM, so no SD wear).

A frame due after `hold` refreshes that lands one or more refreshes
later is "late": the panel repeated the previous frame, a visible hitch.
Gaps of 250ms+ inside a scroll are "freezes" (recomposes, handovers,
blocking calls), counted separately so one handover does not read as 40
missed refreshes. Static frames, the first frame of a scroll and gaps
between scrolls are not timed. The refresh period is estimated from the
frames themselves.

scripts/frame_soak.py runs next to the service as any user, diffs two
snapshots over a run (default 10 minutes), optionally keeps the web
preview's viewer marker fresh, and exits non-zero above 0.1% late
frames. It also reports whether the loaded rgbmatrix binding releases
the GIL. Documented under "Soaking a rig" in docs/SCROLL_PERFORMANCE.md.

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

CLAUDE.md

@@ -34,6 +34,7 @@
 - Browser preview without the display loop: `python3 scripts/dev_server.py` → http://localhost:5001
 - Full display in emulator mode: `python3 run.py -e` (or `EMULATOR=true python3 run.py`)
 - Validate one plugin headlessly: `python3 scripts/check_plugin.py --plugin <id>`
+- Soak a rig for frame timing (on the Pi, service running): `python3 scripts/frame_soak.py --preview` — late-frame rate across every scroller; see `docs/SCROLL_PERFORMANCE.md`
 
 ## Plugin Store Architecture
 - Official plugins live in the `ledmatrix-plugins` monorepo (not individual repos)