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>

hutter/variants/mix/hp_mix.rs

@@ -330,11 +330,11 @@ fn decoder_src_bytes() -> u64 {
 fn report(mode: &str, input: usize, archive: usize, roundtrip: u32) {
     let dsrc = decoder_src_bytes();
     let total = archive as u64 + dsrc;
-    let ratio = if archive > 0 { input as f64 / archive as f64 } else { 0.0 };
-    let bpc = if input > 0 { (archive as f64 * 8.0) / input as f64 } else { 0.0 };
+    let ratio_e4: u128 = if archive > 0 { (input as u128 * 10_000) / archive as u128 } else { 0 }; // integer only (no float)
+    let bpc_e4: u128 = if input > 0 { (archive as u128 * 8 * 10_000) / input as u128 } else { 0 }; // integer only (no float)
     println!(
-        "PILOT|mode={}|input_bytes={}|archive_bytes={}|decoder_src_bytes={}|total_bytes={}|ratio_x={:.4}|bpc={:.4}|roundtrip={}",
-        mode, input, archive, dsrc, total, ratio, bpc, roundtrip
+        "PILOT|mode={}|input_bytes={}|archive_bytes={}|decoder_src_bytes={}|total_bytes={}|ratio_x={}.{:04}|bpc={}.{:04}|roundtrip={}",
+        mode, input, archive, dsrc, total, ratio_e4 / 10_000, ratio_e4 % 10_000, bpc_e4 / 10_000, bpc_e4 % 10_000, roundtrip
     );
 }