Finalize 1.5.3 changelog

Author: eldryoth

CHANGELOG.md

@@ -7,7 +7,7 @@ Fluxheim follows semantic versioning once `1.0.0` is released. Before `1.0.0`,
 minor versions may still change configuration shape, feature names, and runtime
 behavior when the change improves security or project direction.
 
-## 1.5.3 - Unreleased
+## 1.5.3 - 2026-06-04
 
 ### Changed