feat: add record relationships

[dcfocus]

Summary

• add record relationships to Rust/Python DTOs, storage schema, and public exports
• persist relationship edges with migration support for older datasets and related-record lookup
• expose Python add/add_many/search(include_relationships)/related helpers and update docs/tests
• align write batches to evolved dataset schemas and adjust Python tests for MemWAL-visible public reads

Closes #15

Testing

• /home/user/.cargo/bin/cargo fmt --all -- --check
• /home/user/.cargo/bin/cargo clippy --workspace --all-targets -- -D warnings
• /home/user/.cargo/bin/cargo test -p lance-context-core --lib
• /home/user/.cargo/bin/cargo test -p lance-context-core --doc
• /home/user/.local/bin/uv run pytest (96 passed, 4 skipped, 1 xfailed)
• /home/user/.local/bin/uv run ruff format --check python/
• /home/user/.local/bin/uv run ruff check python/
• /home/user/.local/bin/uv run pyright