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.

mempalace/repair.py

@@ -633,6 +633,17 @@ def rebuild_index(
     print(f"{'=' * 55}\n")
     print(f" Palace: {palace_path}")
 
+    # Run the SQLite integrity preflight before any chromadb client open.
+    # ChromaDB's rust binding raises pyo3_runtime.PanicException (which is
+    # not a regular Exception subclass) on a malformed page, propagating
+    # past the try/except around get_collection below. Catching the
+    # corruption here lets us surface the clear recovery instructions and
+    # exit cleanly before chromadb's compactor touches the disk.
+    sqlite_errors = sqlite_integrity_errors(palace_path)
+    if sqlite_errors:
+        print_sqlite_integrity_abort(palace_path, sqlite_errors)
+        return
+
     preflight = maybe_repair_poisoned_max_seq_id_before_rebuild(
         palace_path,
         assume_yes=True,
@@ -676,11 +687,6 @@ def rebuild_index(
         print(e.message)
         return
 
-    sqlite_errors = sqlite_integrity_errors(palace_path)
-    if sqlite_errors:
-        print_sqlite_integrity_abort(palace_path, sqlite_errors)
-        return
-
     # Back up ONLY the SQLite database, not the bloated HNSW files
     sqlite_path = os.path.join(palace_path, "chroma.sqlite3")
     backup_path = sqlite_path + ".backup"