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>

src/lib.rs

@@ -475,16 +475,17 @@ impl OmnibitPixel {
         n_cylinders: usize,
         watcher_bound: u8,
     ) -> Self {
-        let roof = (n_cylinders.min(CYLINDERS.len()) as f64) * 25.0; // ~log2(2^25) per cylinder
-        let block_bits = (8 * BLOCK) as f64;
+        // both quantities are whole bits; integer only (operator rule: no float)
+        let roof: usize = n_cylinders.min(CYLINDERS.len()) * 25; // ~log2(2^25) per cylinder
+        let block_bits: usize = 8 * BLOCK;
         OmnibitPixel {
             x,
             y,
             tick,
             freq,
             digest8: digest8(slice),
-            residual_bits: (block_bits - roof).max(0.0).ceil() as u8,
-            capacity_margin_bits: (roof - block_bits) as i32,
+            residual_bits: block_bits.saturating_sub(roof) as u8,
+            capacity_margin_bits: roof as i32 - block_bits as i32,
             watcher_bound,
         }
     }