feat: add namespace-backed execution flow for lance-graph

[beinan]

Summary

• add execute_with_namespace in the Rust Cypher engine and pull in the lance-namespace crate
• bridge Python namespace objects through a PyNamespaceAdapter and allow CypherQuery.execute to accept catalog objects
• add a pytest covering directory namespaces (skipped when the Lance runtime isn't present) and document the new python/rust dependencies

Testing

• /home/user/.local/bin/uv pip install -e '.[tests]'
• /home/user/.local/bin/uv run pytest python/python/tests/test_graph.py::test_execute_with_directory_namespace -v # skipped: Lance namespace runtime not available