docs: Changelog for 6.6.4 release

Author: DannyvdSluijs

CHANGELOG.md

@@ -6,6 +6,8 @@ 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]
+
+## [6.6.4] - 2025-12-19
 ### Changed
 - ci: Run workflows against 5.x.x branches ([#859](https://github.com/jsonrainbow/json-schema/pull/859))
 ### Fixed