0002: Write everything in Rust, including the numerics

Status: accepted.

Fastmash is written entirely in Rust. The alternative was to keep the numerical kernels in C, using the C library’s long double functions, which would match GNU datamash’s arithmetic most directly. Fastmash implements them in Rust instead, using C only as a comparison baseline during development, so that the most delicate code stays within Rust’s safety checks and the project stays a single-language codebase.

Consequence: Fastmash implements its own 80-bit arithmetic, parsing, formatting and transcendental functions, verified against independent high-precision references. That choice is also what made portable results possible.