fix(display): Vegas resumes after live priority, and six smaller runtime fixes (#644)

- Vegas: a live-priority pause was only lifted from inside run_frame(),
  which returns before that check while paused, so the ticker never came
  back until a restart. run_iteration() now resumes it (the controller
  only calls it when nothing preempts Vegas); start()/stop() clear the
  pause state. Iteration length is timed with the monotonic clock.
- Dim schedule: a per-day disabled day now updates the minute-gate cache,
  so brightness no longer flips back to dim within each minute.
- On-demand: a second request no longer overwrites the rotation resume
  index with the first request's mode.
- Render pipeline: reset() drops the prepared group and deferred queue,
  and a prefetch in flight across a reset discards its result.
- Sync: stop() removes the status file (and the controller's cleanup now
  calls it), standalone removes a stale one at startup, and writes use a
  unique mkstemp temp file.
- render_gate.swap_releases_gil() delegates to frame_timing.
- Stale docstrings/comments corrected.

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

src/display_manager.py

@@ -20,7 +20,10 @@ Key responsibilities
 
 Singleton: only one ``DisplayManager`` instance exists per process.  The
 first call to ``DisplayManager(config)`` creates it; subsequent calls return
-the same object.
+the same object, but ``__init__`` runs again on it each time, so it is
+re-initialised (matrix included) with the new arguments rather than handed
+back as it was.  Construct it once and pass that instance around;
+:meth:`DisplayManager.cleanup` clears the singleton.
 """
 
 import json