toolchain rule: Rust 1.81 + clippy, integer/ternary only

Pin channel 1.81.0 with clippy+rustfmt, rust-version = "1.81", and CI that
runs clippy --all-targets -- -D warnings plus a grep that hard-fails on any
f32/f64. Nothing was pinned before this: no rust-toolchain.toml existed
anywhere in the corpus and 32 different Rust versions were in circulation.

Receipt lines recording past runs above 1.81 are deliberately NOT rewritten
here -- editing a line that records a run would make the receipt false.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: JesseBrown1980