feat(pdo): add next-rowset statement wrapper

[Chemaclass]

Wraps PDOStatement::nextRowset as pdo/next-rowset. Returns the boolean PDO result (true if it advanced to another rowset, false when none remain) - matching the connection-level begin/commit/rollback precedent of returning ^bool rather than the struct, since the boolean is the meaningful signal here.

Note on the test driver

Multi-rowset statements come from stored procedures (MySQL/Postgres). SQLite (the test driver) doesn't support them: nextRowset raises IM001. The test next-rowset-rejected-on-sqlite therefore accepts either false or a \PDOException, proving the call reaches nextRowset. The wrapper is fully functional on drivers that return multiple rowsets.

• Removed from the "Not implemented yet" block in src/pdo/statement.phel.
• README API table + CHANGELOG ## [Unreleased] updated.

Polish pass: no refactor needed - one-liner consistent with the other ^bool mutators.

composer test green (50 assertions).

Closes #17