meta: adopt standard rust/python project layout #18

Summary

Align the repository layout with the standard Lance / PyO3 hybrid structure so the Python package lives under a dedicated python/ directory parallel to Rust crates.

Motivation

• Mirrors the lance-format/lance tree, making it familiar to existing contributors.
• Keeps the Rust core (crates/) free of Python dependencies, enabling reuse in other contexts (WASM, services, etc.).
• Clarifies ownership boundaries between core logic and bindings.

Proposed Layout

/               # workspace root (Cargo.toml)
├── crates/
│   ├── lance-context-core/
│   └── lance-context/
├── python/
│   ├── Cargo.toml         # PyO3 binding crate
│   ├── pyproject.toml     # Maturin build config
│   ├── src/
│   │   └── lib.rs         # #[pymodule] and bridge code
│   └── python/
│       └── lance_context/
│           ├── __init__.py
│           ├── client.py
│           └── schema.py
└── examples/

Details

• Move the existing binding crate and Python package into python/, matching Lance’s monorepo pattern.
• Ensure python/pyproject.toml drives Maturin builds, bundling the Rust glue in python/src and the pure Python code in python/python/lance_context.
• Organize the pure Python layer to hold type hints, async wrappers, and documentation strings that complement the compiled module.

Acceptance Criteria

• Repository adopts the structure above without breaking existing import paths.
• Core crates compile independently of Python.
• Python wheel builds succeed via maturin using the new layout.
• README / contributor docs reflect the new tree.