fix(blob-seq-marker): tests + style nit per @igorls #1177 review

Three new tests cover the marker contract:

- test_fix_blob_seq_ids_writes_marker_after_blob_path — marker is
  written after a successful BLOB → INTEGER conversion.
- test_fix_blob_seq_ids_writes_marker_when_already_integer — marker
  is written even on a no-op (already-INTEGER) path. The point of
  the marker is to skip sqlite3.connect() on subsequent calls, not
  to record that a conversion happened.
- test_fix_blob_seq_ids_skips_sqlite_when_marker_present — verifies
  the load-bearing property via monkeypatched sqlite3.connect: when
  the marker exists, the function never opens sqlite. This is the
  bug #1090 fix — opening Python's sqlite3 against a live ChromaDB
  1.5.x WAL DB corrupts the next PersistentClient call, and we
  must never do it again post-migration.

Also folded in @igorls's style nit:

- Path(marker).touch() instead of open(marker, "a").close().
  Same effect, reads cleaner. Moved Path import to the top of the
  module since it didn't yet exist there.

35/35 backend tests pass. The "Production: fresh MCP server + stop
hook + mempalace mine subprocess" checkbox in the PR body is checked
in the PR reply — the canonical 151K palace has been running this
fix since #1177 was filed (via Syncthing-replicated source on the
disks daemon at v1.7.0); zero PersistentClient corruption since.

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

mempalace/backends/chroma.py

@@ -4,6 +4,7 @@ import datetime as _dt
 import logging
 import os
 import sqlite3
+from pathlib import Path
 from typing import Any, Optional
 
 import chromadb
@@ -206,7 +207,7 @@ def _fix_blob_seq_ids(palace_path: str) -> None:
     # confirmed to be in the INTEGER-seq_id state and future opens can skip the
     # sqlite3.connect() entirely.
     try:
-        open(marker, "a").close()
+        Path(marker).touch()
     except OSError:
         logger.exception("Could not write migration marker %s", marker)
 

tests/test_backends.py

@@ -381,6 +381,75 @@ def test_fix_blob_seq_ids_noop_without_database(tmp_path):
     _fix_blob_seq_ids(str(tmp_path))  # should not raise
 
 
+def test_fix_blob_seq_ids_writes_marker_after_blob_path(tmp_path):
+    """The .blob_seq_ids_migrated marker is written after a successful BLOB → INTEGER conversion."""
+    from pathlib import Path
+    from mempalace.backends.chroma import _BLOB_FIX_MARKER
+
+    db_path = tmp_path / "chroma.sqlite3"
+    conn = sqlite3.connect(str(db_path))
+    conn.execute("CREATE TABLE embeddings (rowid INTEGER PRIMARY KEY, seq_id)")
+    conn.execute("INSERT INTO embeddings (seq_id) VALUES (?)", ((42).to_bytes(8, "big"),))
+    conn.commit()
+    conn.close()
+
+    marker = tmp_path / _BLOB_FIX_MARKER
+    assert not marker.exists()
+
+    _fix_blob_seq_ids(str(tmp_path))
+
+    assert marker.is_file(), "marker must be written after a successful migration"
+
+
+def test_fix_blob_seq_ids_writes_marker_when_already_integer(tmp_path):
+    """The marker is written even when the migration is a no-op (already INTEGER).
+
+    The point of the marker is to skip the sqlite3 open on subsequent calls,
+    not to record that a conversion happened. So a clean palace gets the
+    marker on first run too — next ``_fix_blob_seq_ids`` call short-circuits
+    before touching the sqlite3 file.
+    """
+    from pathlib import Path
+    from mempalace.backends.chroma import _BLOB_FIX_MARKER
+
+    db_path = tmp_path / "chroma.sqlite3"
+    conn = sqlite3.connect(str(db_path))
+    conn.execute("CREATE TABLE embeddings (rowid INTEGER PRIMARY KEY, seq_id INTEGER)")
+    conn.execute("INSERT INTO embeddings (seq_id) VALUES (42)")
+    conn.commit()
+    conn.close()
+
+    marker = tmp_path / _BLOB_FIX_MARKER
+    assert not marker.exists()
+
+    _fix_blob_seq_ids(str(tmp_path))
+
+    assert marker.is_file(), "marker must be written even when no BLOBs found"
+
+
+def test_fix_blob_seq_ids_skips_sqlite_when_marker_present(tmp_path):
+    """When the marker exists, ``_fix_blob_seq_ids`` does not open sqlite3.
+
+    This is the load-bearing property of the marker — opening Python's
+    sqlite3 against a live ChromaDB 1.5.x WAL DB corrupts the next
+    PersistentClient call (#1090). Once a palace has been migrated, we
+    never want to open it again, even read-only.
+    """
+    from pathlib import Path
+    from unittest.mock import patch
+    from mempalace.backends.chroma import _BLOB_FIX_MARKER
+
+    # Pre-create the marker so the function should short-circuit.
+    db_path = tmp_path / "chroma.sqlite3"
+    db_path.write_bytes(b"sentinel")  # presence required for the function to proceed
+    (tmp_path / _BLOB_FIX_MARKER).touch()
+
+    with patch("mempalace.backends.chroma.sqlite3.connect") as mock_connect:
+        _fix_blob_seq_ids(str(tmp_path))
+
+    mock_connect.assert_not_called()
+
+
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