v0.16.2

Cooperative WAL busy-retry: a contended sqlite3step no longer blocks the single cooperative scheduler thread. Native exec/step force busytimeout=0 and surface a 'busy symbol; the public Scheme wrappers retry cooperatively (async-aware sleep suspends the fiber so the lock holder can commit), bounded by a timeout budget so a genuine deadlock errors loudly instead of hanging. Uncontended calls are unchanged.

Validated on sigil 0.17.13: WAL cooperative regression completes in ~207ms (was ~5000ms pre-fix), CRUD suite 10/10. Relocked to build against sigil 0.17.13.