feat(perf): LEDMATRIX_STALL_WATCHDOG_MS lowers the stall watchdog's threshold

250ms catches freezes; the hitches left on hdpi are frames 2-5 refreshes
late, which look like the render thread waiting for the GIL. At 30ms the
watchdog dumps those too, naming what the other threads were running when
the frame missed. It polls at a third of the threshold so a stall one poll
long is still seen, which costs some GIL time of its own: a diagnostic
setting, not one to soak with.

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

src/common/frame_timing.py

@@ -62,7 +62,10 @@ top of every other thread's, so the log names what the render thread was
 waiting on. It also measures how late its own wake-up was: if the watchdog was
 held up as long as the render thread, the whole interpreter was blocked (C
 code holding the GIL, or the process not scheduled), not one thread on a lock.
-Set ``LEDMATRIX_STALL_WATCHDOG=0`` to turn it off.
+Set ``LEDMATRIX_STALL_WATCHDOG=0`` to turn it off, or
+``LEDMATRIX_STALL_WATCHDOG_MS`` to dump at a lower threshold -- 30 catches
+frames three refreshes late, which is where GIL contention shows. It polls
+three times per threshold, so keep it to diagnostic runs, not soaks.
 """
 
 from __future__ import annotations
@@ -283,7 +286,7 @@ class FrameTimingRecorder:
             self._static_frames += 1
         elif self.watchdog is None and self.scrolling_now is not None \
                 and os.environ.get("LEDMATRIX_STALL_WATCHDOG", "1") != "0":
-            self.watchdog = StallWatchdog(self)
+            self.watchdog = StallWatchdog(self, **watchdog_settings())
             self.watchdog.start()
         elif previous is not None and previous[1]:
             interval = presented_at - previous[0]
@@ -417,6 +420,23 @@ class FrameTimingRecorder:
             raise
 
 
+def watchdog_settings() -> Dict[str, float]:
+    """StallWatchdog arguments from ``LEDMATRIX_STALL_WATCHDOG_MS``, if set.
+
+    The poll comes down with the threshold, or a stall shorter than one poll
+    would go unseen.
+    """
+    try:
+        ms = float(os.environ.get("LEDMATRIX_STALL_WATCHDOG_MS") or 0)
+    except ValueError:
+        ms = 0.0
+    if ms <= 0:
+        return {}
+    threshold = ms / 1000.0
+    return {"threshold": threshold,
+            "poll": min(WATCHDOG_POLL_SECONDS, threshold / 3)}
+
+
 class StallWatchdog:
     """Log what the render thread is doing when a scroll stops presenting.