Correct diff masking for historical descendants

docs/roadmap/implementation.md

@@ -248,8 +248,10 @@ Evidence: `go test ./internal/engine -run 'Diff|Effective|Snapshot'` and
 `./scripts/check.sh` passed. Coverage includes both mapping/scalar directions,
 mapping/sequence replacements with numeric path tokens, null/container changes,
 nested sequence mismatches, historical-only descendant classification,
-unrelated sensitive siblings, targeted masking through absence, and fallback
-preserving an absent reported side.
+unrelated sensitive siblings, and targeted masking through absence. Post-review
+remediation corrects the original stronger-policy tests: historical descendants
+alone do not mask same-kind scalars or public additions across absence. A
+historical-only selector still triggers fallback for incompatible present kinds.
 
 ## Stage 6 — Distinguish hidden changes from unchanged masked content
 
@@ -279,6 +281,14 @@ unchanged masked mapping and sequence members, masked additions/removals,
 sequence-length changes, structural fallback, and equal values with differing
 classification but no changed region.
 
+Post-review regressions also verify public-only sequence changes alongside an
+unchanged scalar with historical sensitive descendants, plus public scalar
+addition/removal across historical absence. These retain visible snapshots and
+`HiddenChanged=false` while historical explanation sensitivity remains intact.
+The corrected tests failed before remediation and passed after removing the
+extra scalar/default fallback. Focused diff checks, `./scripts/check.sh`, and
+`go test -race ./...` passed.
+
 ## Stage 7 — Assemble the complete engine diff operation
 
 Status: complete. Scope: internal-facing records, serialization, and real loads.