fix(vegas): use a monotonic clock for the frame-rate timers

Self-review catch. The heartbeat added in this PR compared wall-clock
timestamps, which is the defect CodeRabbit flagged on the metrics throttle and
which I had already fixed there: these devices have no RTC, so the clock jumps
by however wrong boot time was when NTP first syncs. A backward jump would
suppress the heartbeat, a forward one fire it early.

The same value also divides the frame count to produce the frame rate, so a
jump corrupted the reported fps as well -- a pre-existing problem this makes
worth fixing rather than working around.

last_fps_log_time was seeded from start_time, which is wall clock and is used
further down to report the iteration duration. Switching only the reads would
have made every delta hugely negative and silenced frame-rate reporting
completely, so the seed moves to time.monotonic() and start_time is left alone
for the duration reporting it exists for.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW

src/vegas_mode/coordinator.py

@@ -406,7 +406,11 @@ class VegasModeCoordinator:
         fps_log_interval = 5.0  # Sample FPS every 5 seconds
         last_fps_health_log = 0.0   # last INFO-level report
         was_degraded = False        # so the recovery is reported too
-        last_fps_log_time = start_time
+        # Monotonic, and deliberately not start_time: start_time is wall
+        # clock and is used below to report the iteration's duration. Mixing
+        # the two here would make every delta hugely negative and silence the
+        # frame-rate reporting altogether.
+        last_fps_log_time = time.monotonic()
         fps_frame_count = 0
         # A mean hides stutter completely. At 120fps a five-second window is
         # ~600 frames, so a 200ms freeze -- plainly visible on a marquee --
@@ -467,7 +471,12 @@ class VegasModeCoordinator:
             # of them within 10% of target. The 1.5% that were not included a
             # reading of 8.6fps against a target of 60 -- a real stall, and
             # completely invisible inside 1389 lines reading "59.6".
-            current_time = time.time()
+            # Monotonic: every use of this value in the block below is a
+            # duration, and these devices have no RTC, so the wall clock jumps
+            # by however wrong boot time was the moment NTP first syncs. That
+            # would not only mis-fire the heartbeat, it would corrupt the
+            # frame rate itself, since fps is frames divided by this delta.
+            current_time = time.monotonic()
             if current_time - last_fps_log_time >= fps_log_interval:
                 fps = fps_frame_count / (current_time - last_fps_log_time)
                 p99 = _percentile(sorted(frame_times), 0.99)