docs(CHANGES): document fmt snapshot coverage

Author: tony

CHANGES

@@ -41,6 +41,8 @@ _Upcoming changes will be written here._
   formatting so users keep a single normalized section.
 - Adds `--no-merge` for workflows that need to preserve duplicate entries while
   still seeing diagnostic warnings.
+- Tests: Formatter coverage now uses Syrupy JSON/YAML snapshots to guard
+  against regressions in duplicate merge behavior.
 
 ## vcspull v1.42.0 (2025-11-01)