fix(hnsw): gate quarantine_stale_hnsw to cold-start, not every reconnect

Symptom on the canonical disks daemon: drift quarantines firing every
10–30 minutes throughout the day under steady write load. Logs show
.drift-* directories accumulating despite the daemon being the only
writer (no Syncthing replication of palace data).

Root cause is a false-positive thrash in the quarantine heuristic:

- chroma.sqlite3 mtime bumps on every write (millisecond cadence).
- HNSW segment files (data_level0.bin) only flush to disk on
  chromadb's internal cadence, which can lag minutes behind sqlite
  under continuous write load.

Once the gap exceeds the 300s threshold, quarantine_stale_hnsw renames
a perfectly valid HNSW segment, chromadb rebuilds it from scratch, and
the cycle repeats as soon as the next batch of writes lands. The 300s
threshold (lowered from 3600s in PR #1173 after a 0.96h-drift production
segfault) is correct for the cross-machine-replication failure mode it
was designed for, but wrong for a daemon-strict deployment whose only
"drift" source is its own benign flush lag.

Fix: gate the proactive quarantine check to the first ``make_client()``
invocation per palace per process (``ChromaBackend._quarantined_paths``
set). Real cold-start drift (replication, partial restore, crashed-mid-
write) still gets caught — that's exactly when a fresh daemon process
opens the palace. Real runtime drift on observed HNSW errors still gets
caught via palace-daemon's ``_auto_repair`` which calls
``quarantine_stale_hnsw`` directly, bypassing this gate.

Two new tests in test_backends.py verify single-fire-per-palace and
per-palace independence. Conftest clears the gate between tests.

Suite 1362/1362 pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/conftest.py

@@ -46,6 +46,14 @@ def _reset_mcp_cache():
             mcp_server._collection_cache = None
         except (ImportError, AttributeError):
             pass
+        try:
+            # Reset the per-process quarantine gate so tests don't leak
+            # state through ChromaBackend._quarantined_paths.
+            from mempalace.backends.chroma import ChromaBackend
+
+            ChromaBackend._quarantined_paths.clear()
+        except (ImportError, AttributeError):
+            pass
 
     _clear_cache()
     yield