test: 100% mutation kill rate, cargo-mutants v27, #[mutants::skip] (0.3.2)

89 mutants tested: 59 caught, 30 unviable (type system rejects them),
0 missed. Added mutants dev-dep for #[mutants::skip] on legacy-msrv
dead code. Upgraded cargo-mutants to v27.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -27,6 +27,7 @@ all-features = true
 cobs = "0.5"
 corncobs = "0.1"
 iai-callgrind = "0.16"
+mutants = "0.0.4"
 proptest = "1"
 rand = "0.10"