fix(state): guard gateway FTS rebuild + comment early flag-set

Add the foreign-holder guard to gateway/session.py::_rebuild_fts_once(),
the third FTS rebuild path that was not covered by the original fix.
Also add a comment explaining why _fts_runtime_rebuild_attempted is set
before the foreign-holder check: the fail-open path that follows
persists FTS_STALE_KEY so the next startup retries via _recover_stale_fts.

gateway/session.py

@@ -3875,6 +3875,19 @@ class SessionStore:
         db = self._db
         if db is None or not hasattr(db, "rebuild_fts"):
             return False
+        # Guard against the same WAL split-brain risk as the automatic
+        # rebuild paths: skip when a foreign process holds state.db or
+        # its WAL sidecars open.
+        if hasattr(db, "_foreign_state_db_holders"):
+            foreign_holders = db._foreign_state_db_holders()
+            if foreign_holders:
+                logger.warning(
+                    "Skipping Session DB FTS rebuild while foreign processes "
+                    "hold the database or WAL sidecars (%s); canonical "
+                    "transcript writes remain available.",
+                    foreign_holders,
+                )
+                return False
         try:
             rebuilt = db.rebuild_fts()
         except Exception as exc: