The original installation does not work properly:
- `uv pip show nanochat` worked but importing from a script outside
the nanochat directory failed.
- rustbpe got installed as a module not a package.
Fix by creating a uv workspace with:
- nanochat: Python package (uv_build backend, depends on rustbpe)
- rustbpe: Rust extension package (maturin backend)
- moving nanochat to src/, as required by uv.
Also updates speedrun.sh, dependency versions, and cleans trailing
whitespace.
