docs(CHANGES): document fmt snapshot coverage

Author: tony

CHANGES

@@ -42,6 +42,15 @@ _Upcoming changes will be written here._
 - Adds `--no-merge` for workflows that need to preserve duplicate entries while
   still seeing diagnostic warnings.
 
+### Development
+
+#### Snapshot coverage for formatter tests (#479)
+
+- Formatter scenarios now use [Syrupy]-backed JSON and YAML snapshots to guard
+  against regressions in duplicate workspace-root merging.
+
+[syrupy]: https://github.com/syrupy-project/syrupy
+
 ## vcspull v1.42.0 (2025-11-01)
 
 ### Breaking changes