feat(display): report a scrolling screen held by its plugin's update() (#758)

While a plugin's update() runs it holds the plugin's lock and its frames are skipped -- on a scroller, a frozen strip -- with nothing logged. The high-FPS loop now times each run of skipped frames (report_hold=True); one of 250 ms or more logs 'Display of X held N ms by its update()' (rate-limited per plugin) and is recorded as a 'display hold' busy skip, which never touches the circuit breaker. The 1 Hz loop is left out: one skipped frame there measures the loop interval on a screen that did not visibly freeze (seen on ledpi as ~1000 ms reports on clock-simple and switch-mode football).

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

CHANGELOG.md

@@ -19,6 +19,18 @@ accepts both, but the store flags the old spelling as deprecated
 
 ## Unreleased
 
+### A scrolling screen held by its plugin's update() is reported
+
+- While a plugin's `update()` runs it holds the plugin's lock, and that
+  plugin's frames are skipped: on a scroller, a frozen strip, with nothing
+  logged (and a freeze of 5 s or more is a gap, not a freeze, to the frame
+  stats). The high-FPS loop now times each run of skipped frames; one of
+  250 ms or more logs `Display of <plugin> held N ms by its update()`
+  (rate-limited per plugin) when it ends, and is recorded on the plugin's
+  health as a `display hold` busy skip, which never counts toward the
+  circuit breaker. The 1 Hz loop is left out: its frames are a second apart,
+  so one skipped frame there measures nothing and freezes nothing visible.
+
 ### Fixed
 
 - The web preview and `/api/v3/display/current` no longer stay black for a