fix(frame-timing): keep the GC monitor quiet at interpreter shutdown (#734)

A collection during interpreter shutdown called GcMonitor after the
module's `time` global was torn down, printing "Exception ignored while
calling GC callback ... 'NoneType' object has no attribute
'perf_counter'" at the end of service and test runs.

- GcMonitor binds its clock and sys.is_finalizing at construction and
  does nothing once the interpreter is finalizing.
- install_gc_monitor() unregisters it with atexit; new
  uninstall_gc_monitor().
- DisplayManager.cleanup() (reached from SIGTERM via run()'s finally)
  unregisters it alongside the frame recorder.

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

src/display_manager.py

@@ -57,7 +57,8 @@ import freetype
 
 from src.common import snapshot_policy
 from src import display_watchdog
-from src.common.frame_timing import FrameTimingRecorder, install_gc_monitor
+from src.common.frame_timing import (
+    FrameTimingRecorder, install_gc_monitor, uninstall_gc_monitor)
 
 if TYPE_CHECKING:
     from src.common.render_gate import RenderGate
@@ -1407,6 +1408,8 @@ class DisplayManager:
         # The stall watchdog would otherwise outlive this manager.
         if getattr(self, 'frame_timing', None) is not None:
             self.frame_timing.close()
+            # Installed with the recorder; stop timing collections with it.
+            uninstall_gc_monitor()
         # Reset the singleton state when cleaning up
         DisplayManager._instance = None