Nested transaction() throws: cannot start a transaction within a transaction #17

tests/integration/graph-operations.test.ts › "should support nested transaction-like operations" fails on main (a7f9ea7) with:

SqliteError: cannot start a transaction within a transaction
  at GraphDatabase.transaction (src/core/Database.ts)

transaction() issues a plain BEGIN. The test calls db.transaction() inside another db.transaction(). Supporting this likely means using a SAVEPOINT when a transaction is already open (db.inTransaction).

This suite never ran far enough to show the failure before #10 (it ran out of memory first).