fix(display): serialize update_display, narrow brightness exception, log fixes

CodeRabbit review on #406, verified against current code:

- update_display() can genuinely be called from background threads (some
  sports base classes call it directly from inside update() for an
  immediate "live" refresh), not just the render loop — confirmed via the
  existing follower-mode gating wrapper in display_controller.py, which
  exists specifically because "background plugin threads" can reach it.
  Without a lock, two callers could both pass the digest check before
  either writes _last_pushed_digest back, causing a redundant push, or
  interleave the offscreen/current canvas swap. Added self._update_lock
  (RLock, in case of re-entrant callers) around the full method body so
  every call site is automatically covered — no caller changes needed.
  (No prior lock existed to reuse on DisplayManager; this adds one.)
- Narrowed the brightness-read exception handler to AttributeError,
  matching the established pattern in get_brightness()/set_brightness()
  — a getattr() with a default already swallows AttributeError, so the
  only case this guards is the property getter itself raising, and the
  established pattern treats that as an expected, specific failure mode
  rather than something to blanket-catch.
- FPS-check debug log now includes the plugin_id already in scope
  (previously only active_mode) and a "[DisplayController]" prefix for
  grep-ability, matching the sibling log two lines below it.
- test_display_dirty_tracking.py: dm fixture and test_config_flag_wires_through
  now reset the DisplayManager singleton on teardown, matching the pattern
  test_display_manager.py already uses elsewhere in the same file family.
- test_snapshot_still_written_on_skip previously only exercised the
  non-skip (push) path despite its name; now performs a second update that
  meets the skip conditions (identical frame) and asserts the snapshot is
  still written even though the panel push itself is skipped.

All 7 dirty-tracking tests pass, plus the full display_manager/
display_controller/vegas suite (140 passed). Full repo suite has only the
5 known pre-existing failures (double-sided config x2, state_reconciliation
x2, and test_circuit_breaker's conftest.py mock signature drift — the
latter fixed in #400, which this branch's base predates).

test/test_display_dirty_tracking.py

@@ -36,6 +36,8 @@ def dm():
         },
     }, suppress_test_pattern=True)
     yield manager
+    DisplayManager._instance = None
+    DisplayManager._initialized = False
 
 
 class _SwapSpy:
@@ -101,6 +103,16 @@ class TestDirtyTracking:
         dm.update_display()   # push + snapshot
         assert os.path.exists(dm._snapshot_path)
 
+        # Re-open the snapshot's own throttle and remove the file, then push
+        # the identical frame again: dirty tracking must skip the panel
+        # write but the snapshot must still be (re-)written on that path.
+        os.remove(dm._snapshot_path)
+        dm._last_snapshot_ts = 0.0
+        with _SwapSpy(dm.matrix) as spy:
+            dm.update_display()  # identical frame -> panel push skipped
+        assert spy.count == 0
+        assert os.path.exists(dm._snapshot_path)
+
 
 class TestKillSwitch:
     def test_dirty_tracking_can_be_disabled(self, dm):
@@ -120,15 +132,19 @@ class TestKillSwitch:
         from src.display_manager import DisplayManager
         DisplayManager._instance = None
         DisplayManager._initialized = False
-        manager = DisplayManager({
-            "display": {
-                "hardware": {"rows": 32, "cols": 64, "chain_length": 1,
-                             "parallel": 1},
-                "runtime": {"gpio_slowdown": 0},
-                "dirty_tracking": False,
-            },
-        }, suppress_test_pattern=True)
-        assert manager._dirty_tracking_enabled is False
+        try:
+            manager = DisplayManager({
+                "display": {
+                    "hardware": {"rows": 32, "cols": 64, "chain_length": 1,
+                                 "parallel": 1},
+                    "runtime": {"gpio_slowdown": 0},
+                    "dirty_tracking": False,
+                },
+            }, suppress_test_pattern=True)
+            assert manager._dirty_tracking_enabled is False
+        finally:
+            DisplayManager._instance = None
+            DisplayManager._initialized = False
 
 
 if __name__ == "__main__":