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>

CHANGELOG.md

@@ -1,6 +1,14 @@
 # Changelog
 
-## 0.3.2 — 2026-04-01
+## 0.3.3 — 2026-04-01
+
+### Fixed
+
+- **Fix compilation for downstream users** — `#[mutants::skip]` attribute was
+  not gated behind `cfg(test)`, causing unresolved module error when ucobs is
+  used as a dependency.
+
+## 0.3.2 — 2026-04-01 (yanked)
 
 ### Testing
 

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ucobs"
-version = "0.3.2"
+version = "0.3.3"
 edition = "2021"
 rust-version = "1.93"
 license = "MIT"

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) {