feat(namespace): add directory-backed execution

[beinan]

Summary

• add a native DirNamespace resolver in Rust and surface it through the Python bindings
• keep the dictionary-based execute path and add execute_with_namespace
• cover namespace execution with a Lance-backed pytest

Testing

• uv run maturin develop
• uv run pytest python/python/tests/test_graph.py::test_execute_with_directory_namespace -v
  Closes Feature Request: Support lance-namespace for Automatic Table Resolution in CypherQuery.execute() #90.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.54430% with 39 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-graph/src/query.rs	82.35%	33 Missing ⚠️
rust/lance-graph/src/namespace/directory.rs	88.00%	6 Missing ⚠️

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