cm: update CHANGELOG for SetOK and SetErr

Author: ydnar

cm/CHANGELOG.md

@@ -2,6 +2,12 @@
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+
+- Mutating methods `SetOK` and `SetErr` on `result` types (`Result[Shape, OK, Err]`).
+
 ## [v0.2.2] — 2025-03-16
 
 ### Fixed