add ecdhmask changelog entry (OpenZeppelin#677)

Author: andrew-fleming

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Added
+
+- Add EcdhMask (#655)
+
 ## 0.3.0-alpha (2026-06-30)
 
 ### Added