feat: iterate rows with `foreach ($matrix as $row)` via ObjectGetIteratorInterface

[lisachenko]

z-engine dev-master bridges the engine's get_iterator handler to userland iterators. Implementing ObjectGetIteratorInterface::__getIterator() returning an iterator over rows would make foreach ($matrix as $i => $row) work without implementing Traversable on the class.

Contract notes from z-engine: by-reference iteration is rejected by the bridge, and iterator methods must not throw across the FFI boundary (errors surface as E_USER_WARNING and iteration terminates) — keep the iterator allocation-only over the already-validated internal array.