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>

test/test_display_controller_schedule.py

@@ -254,6 +254,27 @@ class TestDimSchedule:
         assert dim_at(dc, "23:00", day="tuesday") == 90  # day disabled
         assert dc.is_dimmed is False
 
+    def test_disabled_day_stays_normal_within_the_minute(self):
+        # Dim late Monday, then Tuesday (dimming disabled) begins. The second
+        # call in the same minute is served from the minute-gate cache, which
+        # the disabled-day branch used to leave holding Monday's dim value --
+        # so brightness flipped back to dim for the rest of every minute.
+        dc = make_controller(self._config(mode="per-day", days={
+            "monday": {"enabled": True, "start_time": "22:00",
+                       "end_time": "06:00"},
+            "tuesday": {"enabled": False},
+        }))
+        assert dim_at(dc, "23:59", day="monday") == 25
+        dc._dim_checked_minute = None
+        p = at("00:00", day="tuesday")
+        try:
+            assert dc._check_dim_schedule() == 90
+            assert dc._check_dim_schedule() == 90  # cached, same minute
+        finally:
+            p.stop()
+        assert dc.is_dimmed is False
+        assert dc._was_dimmed is False
+
     def test_no_legacy_inference_for_dim(self):
         # Unlike _check_schedule, dim mode defaults to GLOBAL even when a
         # days config exists — no legacy inference.