Rans64Encoder/Rans64Decoder use u64 state and emit u32 words during
renormalization. This gives finer frequency resolution and fewer renorm
steps per symbol -- the variant used by JPEG XL, LZFSE, and zstd.

- Rans64Encoder: put, finish, state (bits-back)
- Rans64Decoder: get, peek, advance, state (bits-back)
- Batch helpers: encode64, decode64
- 7 new tests including proptest roundtrip
- README: document 64-bit API, note WASM compatibility