fix: gate #[mutants::skip] behind cfg(test) for downstream compat (0.3.3)

The #[mutants::skip] proc macro from the mutants dev-dep was not available
to downstream users, causing compilation failure. Gate it with cfg_attr(test).

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

src/lib.rs

@@ -167,7 +167,7 @@ const fn const_copy(dest: &mut [u8], di: usize, src: &[u8], n: usize) {
 }
 
 /// Fallback const copy for toolchains older than Rust 1.93.
-#[mutants::skip] // dead code under default features; tested via --features legacy-msrv
+#[cfg_attr(test, mutants::skip)] // dead code under default features
 #[inline(always)]
 #[cfg(feature = "legacy-msrv")]
 const fn const_copy(dest: &mut [u8], di: usize, src: &[u8], n: usize) {