fix(repair): run SQLite integrity preflight before chromadb open

#1364 added the SQLite quick_check preflight to rebuild_index, but
placed it AFTER backend.get_collection(...). On a SQLite-corrupt
palace, chromadb's rust binding raises pyo3_runtime.PanicException —
which is not a regular Exception subclass — so it propagates past the
existing `except Exception` handlers and the user sees a 30-line stack
trace instead of the friendly abort message #1364 was designed to
deliver. Reproduced with `mempalace repair --yes` against a palace
whose chroma.sqlite3 has 4 mangled pages: pre-fix, panic; post-fix,
the clean abort message and exit code 1.

Two changes:

- mempalace/cli.py cmd_repair: run sqlite_integrity_errors() right
  after the basic palace-existence check, BEFORE the max_seq_id
  preflight (which itself opens sqlite3) and BEFORE backend =
  ChromaBackend(). Exit non-zero so unattended scripts and CI gates
  see the failure.

- mempalace/repair.py rebuild_index: same move at the function level
  for direct callers (tests, MCP) that bypass cmd_repair.

The new test test_rebuild_index_runs_sqlite_preflight_before_chromadb_open
uses a real chromadb-built palace (no ChromaBackend mock) plus a
real corrupt SQLite (16 KB of mangled pages) so the ordering is
exercised end-to-end. The previously-shipping test for the abort path
mocked both the backend and sqlite_integrity_errors, which is why the
ordering bug shipped CI-green.

Six existing test_cli.py cmd_repair tests used `(palace_dir /
"chroma.sqlite3").write_text("db")` to fake the SQLite file. The new
preflight correctly fails quick_check on those 2-byte stubs, so the
tests now create empty real SQLite DBs the same way the test_repair.py
fixtures already do.

tests/test_cli.py

@@ -2,6 +2,7 @@
 
 import argparse
 import shlex
+import sqlite3
 import sys
 from pathlib import Path
 from unittest.mock import MagicMock, call, patch
@@ -774,7 +775,7 @@ def test_cmd_repair_requires_palace_database(mock_config_cls, tmp_path, capsys):
 def test_cmd_repair_error_reading(mock_config_cls, tmp_path, capsys):
     palace_dir = tmp_path / "palace"
     palace_dir.mkdir()
-    (palace_dir / "chroma.sqlite3").write_text("db")
+    sqlite3.connect(str(palace_dir / "chroma.sqlite3")).close()
     mock_config_cls.return_value.palace_path = str(palace_dir)
     mock_config_cls.return_value.collection_name = "mempalace_drawers"
     args = argparse.Namespace(palace=None)
@@ -790,7 +791,7 @@ def test_cmd_repair_error_reading(mock_config_cls, tmp_path, capsys):
 def test_cmd_repair_zero_drawers(mock_config_cls, tmp_path, capsys):
     palace_dir = tmp_path / "palace"
     palace_dir.mkdir()
-    (palace_dir / "chroma.sqlite3").write_text("db")
+    sqlite3.connect(str(palace_dir / "chroma.sqlite3")).close()
     mock_config_cls.return_value.palace_path = str(palace_dir)
     mock_config_cls.return_value.collection_name = "mempalace_drawers"
     args = argparse.Namespace(palace=None)
@@ -807,7 +808,7 @@ def test_cmd_repair_zero_drawers(mock_config_cls, tmp_path, capsys):
 def test_cmd_repair_success(mock_config_cls, tmp_path, capsys):
     palace_dir = tmp_path / "palace"
     palace_dir.mkdir()
-    (palace_dir / "chroma.sqlite3").write_text("db")
+    sqlite3.connect(str(palace_dir / "chroma.sqlite3")).close()
     mock_config_cls.return_value.palace_path = str(palace_dir)
     mock_config_cls.return_value.collection_name = "mempalace_drawers"
     args = argparse.Namespace(palace=None, yes=True)
@@ -843,7 +844,7 @@ def test_cmd_repair_success(mock_config_cls, tmp_path, capsys):
 def test_cmd_repair_uses_configured_collection(mock_config_cls, tmp_path, capsys):
     palace_dir = tmp_path / "palace"
     palace_dir.mkdir()
-    (palace_dir / "chroma.sqlite3").write_text("db")
+    sqlite3.connect(str(palace_dir / "chroma.sqlite3")).close()
     mock_config_cls.return_value.palace_path = str(palace_dir)
     mock_config_cls.return_value.collection_name = "custom_drawers"
     args = argparse.Namespace(palace=None, yes=True)
@@ -882,7 +883,7 @@ def test_cmd_repair_uses_configured_collection(mock_config_cls, tmp_path, capsys
 def test_cmd_repair_restores_backup_on_live_rebuild_failure(mock_config_cls, tmp_path, capsys):
     palace_dir = tmp_path / "palace"
     palace_dir.mkdir()
-    (palace_dir / "chroma.sqlite3").write_text("db")
+    sqlite3.connect(str(palace_dir / "chroma.sqlite3")).close()
     mock_config_cls.return_value.palace_path = str(palace_dir)
     mock_config_cls.return_value.collection_name = "mempalace_drawers"
     args = argparse.Namespace(palace=None, yes=True)
@@ -916,7 +917,7 @@ def test_cmd_repair_restores_backup_on_live_rebuild_failure(mock_config_cls, tmp
 def test_cmd_repair_aborts_without_confirmation(mock_config_cls, tmp_path, capsys):
     palace_dir = tmp_path / "palace"
     palace_dir.mkdir()
-    (palace_dir / "chroma.sqlite3").write_text("db")
+    sqlite3.connect(str(palace_dir / "chroma.sqlite3")).close()
     mock_config_cls.return_value.palace_path = str(palace_dir)
     mock_config_cls.return_value.collection_name = "mempalace_drawers"
     args = argparse.Namespace(palace=None)