analyze: walk audit chains read-only — no migration DDL on a read op

`arborist analyze`'s per-shard audit-chain loop opened each shard with
connect(), which on first open of a file runs executescript(SCHEMA_SQL)
plus all forward migrations — DDL on a read-only walk, on 6 GB shards,
and (until 71c9848) the trigger for the fork_score_branches crash.

New store.connect_readonly(db_path): mode=ro URI, no schema bootstrap,
no migration probes. _cmd_analyze's chain-check loop uses it.

arborist/cli.py

@@ -18,6 +18,7 @@ from arborist.store import (
     append_audit,
     connect,
     connect_query,
+    connect_readonly,
     stats,
     transaction,
 )
@@ -2934,7 +2935,9 @@ def _cmd_analyze(args: argparse.Namespace) -> int:
         total_events = 0
         total_breaks = 0
         for sp in discover_shards(args.global_shards_dir):
-            sc = connect(sp)
+            # Read-only: an audit-chain walk must not run migration DDL
+            # or take a write lock on the shard (see connect_readonly).
+            sc = connect_readonly(sp)
             try:
                 ev, br = _check_audit_chain(sc)
             finally:

arborist/store.py

@@ -1475,6 +1475,27 @@ def connect_query(
     return conn
 
 
+def connect_readonly(db_path: Path | str) -> sqlite3.Connection:
+    """Open an existing DB read-only — ``mode=ro`` URI, no schema
+    bootstrap, no migration probes.
+
+    For CLI paths that only walk data (audit-chain checks, per-shard
+    counts): a read op must neither run DDL nor take a write lock, but
+    ``connect()`` does both — it runs ``executescript(SCHEMA_SQL)`` plus
+    the forward migrations on the first open of each file in a process.
+    (That migration pass on a read-only walk is exactly what surfaced
+    the ``fork_score_branches already exists`` crash.) Raises
+    ``sqlite3.OperationalError`` if the file is missing or unreadable;
+    callers iterating a shard set should catch and skip if they expect
+    stragglers.
+    """
+    conn = sqlite3.connect(
+        f"file:{Path(db_path)}?mode=ro", uri=True, isolation_level=None
+    )
+    conn.row_factory = sqlite3.Row
+    return conn
+
+
 @contextmanager
 def transaction(conn: sqlite3.Connection) -> Iterator[sqlite3.Connection]:
     """BEGIN IMMEDIATE / COMMIT / ROLLBACK around a block."""