v0.1.1

Add batch ratio API (parallel inside Rust, GIL released) + v0.1.1

ratio is now overloaded from Python:
  ratio(a, b)     -> float         (one pair)
  ratio(pairs)    -> list[float]   (parallel across all cores via rayon, GIL released)

The list form is the contention-free way to use every core from Python — the
fan-out happens in Rust, no ThreadPoolExecutor and no GIL fight. Backed by the
new public Rust fn ratio_many; scalar ratio and the cluster fns also release the
GIL. Typed via @overload in the stubs (pyright verifies the scalar/list split).

Adds benchmarks/bench_python.py (correctness + single/batch/cluster throughput
vs stdlib difflib, and the difflib-in-threads-doesn't-scale contrast). README
Python section rewritten around the import-and-it's-faster hook + the numbers.

Bumps to 0.1.1.