fix(core): harden text-measurement caches; surface snapshot failures (#400)

* fix(core): harden text-measurement caches; surface snapshot failures

Deep-dive findings, all three latent on every 24/7 install:

- font_manager.metrics_cache and display_manager._text_width_cache were
  unbounded dicts keyed by (text, id(font)). Two problems: keys embed
  the measured TEXT, so ever-changing strings (a clock, a live score, a
  ticker) grow them without limit; and id()-keying without holding a
  reference means a garbage-collected font's id can be recycled by a
  DIFFERENT font, silently returning wrong widths/metrics (classic
  plugins create fonts per render, so this is reachable). Both caches
  are now LRU-bounded (1024) and pin the font in the entry so its id
  stays valid. metrics_cache also keyed on the text itself instead of
  hash(text), removing a collision path.

- _write_snapshot_if_due logged failures at DEBUG — invisible at the
  default level. The snapshot's mtime is the web UI's display mirror
  AND its hardware-liveness proxy, so a quiet failure freezes the
  mirror and makes health checks lie (seen in the field: a stale
  root-owned /tmp file froze it for a day). Failures now WARN, rate-
  limited to once per 5 minutes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FqzC1nzTWL4kaqgMaQZFam

* test: sync mock cache manager signature with CacheManager.get

test_circuit_breaker has been failing on main: plugin_health passes
memory_ttl= to cache_manager.get(), and the conftest mock's signature
was never updated — the same component/double drift class as the
monitored_update bug (#392).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FqzC1nzTWL4kaqgMaQZFam

---------

Co-authored-by: Chuck <chuck@example.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

test/conftest.py

@@ -38,7 +38,11 @@ def mock_cache_manager():
     mock._memory_cache_timestamps = {}
     mock.cache_dir = "/tmp/test_cache"
     
-    def mock_get(key: str, max_age: int = 300) -> Optional[Dict]:
+    def mock_get(key: str, max_age: Optional[int] = 300,
+                 memory_ttl: Optional[int] = None) -> Optional[Dict]:
+        # Signature mirrors CacheManager.get — keep in sync or callers
+        # passing keyword args (health tracker, resource monitor) break
+        # only in tests, hiding real-API compatibility.
         return mock._memory_cache.get(key)
     
     def mock_set(key: str, data: Dict, ttl: Optional[int] = None) -> None: