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