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

tests/test_backends.py

@@ -1,5 +1,6 @@
 import os
 import sqlite3
+from pathlib import Path
 
 import chromadb
 import pytest
@@ -446,6 +447,69 @@ def test_quarantine_stale_hnsw_skips_already_quarantined(tmp_path):
     assert drift.exists()
 
 
+# ── make_client cold-start gate ──────────────────────────────────────────
+
+
+def test_make_client_quarantines_only_on_first_call_per_palace(tmp_path, monkeypatch):
+    """Quarantine fires on first ``make_client()`` for a palace, then is
+    skipped on subsequent calls — prevents runtime thrash where a daemon's
+    own steady writes bump ``chroma.sqlite3`` faster than HNSW flushes,
+    making the mtime heuristic falsely trigger every reconnect."""
+    from mempalace.backends.chroma import ChromaBackend
+
+    palace_path = str(tmp_path / "palace")
+    os.makedirs(palace_path, exist_ok=True)
+    (Path(palace_path) / "chroma.sqlite3").write_text("")
+
+    # Reset the per-process cache so this test is independent of others.
+    monkeypatch.setattr(ChromaBackend, "_quarantined_paths", set())
+
+    calls: list[str] = []
+
+    def _spy(path, stale_seconds=300.0):
+        calls.append(path)
+        return []
+
+    monkeypatch.setattr("mempalace.backends.chroma.quarantine_stale_hnsw", _spy)
+
+    ChromaBackend.make_client(palace_path)
+    ChromaBackend.make_client(palace_path)
+    ChromaBackend.make_client(palace_path)
+
+    assert calls == [palace_path], (
+        "quarantine_stale_hnsw should fire once per palace per process, not on every reconnect"
+    )
+
+
+def test_make_client_quarantines_each_palace_independently(tmp_path, monkeypatch):
+    """Two distinct palaces each get one quarantine attempt — the gate is
+    keyed by palace path, not global."""
+    from mempalace.backends.chroma import ChromaBackend
+
+    palace_a = str(tmp_path / "palace_a")
+    palace_b = str(tmp_path / "palace_b")
+    for p in (palace_a, palace_b):
+        os.makedirs(p, exist_ok=True)
+        (Path(p) / "chroma.sqlite3").write_text("")
+
+    monkeypatch.setattr(ChromaBackend, "_quarantined_paths", set())
+
+    calls: list[str] = []
+
+    def _spy(path, stale_seconds=300.0):
+        calls.append(path)
+        return []
+
+    monkeypatch.setattr("mempalace.backends.chroma.quarantine_stale_hnsw", _spy)
+
+    ChromaBackend.make_client(palace_a)
+    ChromaBackend.make_client(palace_b)
+    ChromaBackend.make_client(palace_a)  # already gated
+    ChromaBackend.make_client(palace_b)  # already gated
+
+    assert calls == [palace_a, palace_b]
+
+
 # ── _pin_hnsw_threads ─────────────────────────────────────────────────────