The entry_data/lock_held path (used by mark_turn_active/clear_turn_active)
was eagerly constructing a full O(n) snapshot of all routing entries under
the lock on every turn, even when the DB upsert would succeed. Defer the
fallback_data construction to the except branch where it's actually needed,
and respect lock_held to avoid re-acquiring a non-reentrant _lock.
