US-6.2.1: Python bindings for structures and properties (PyO3)

## User Story
> As a Python chemist, I want to use polysim from Python.

## Acceptance Criteria
- [ ] `pip install polysim`
- [ ] `from polysim import PolymerChain, LinearBuilder, BuildStrategy`
- [ ] Pythonic API (snake_case, docstrings, type hints)
- [ ] NumPy support for 3D coordinates
- [ ] RDKit integration (SMILES <-> Mol conversion)

## New Crate
`crates/polysim-python/`