Merge provider defaults into one owned copy

docs/roadmap/implementation.md

@@ -397,6 +397,13 @@ disjoint keys, nulls, arrays, and input mutation. Repeat the roadmap's chain/
 width/disjoint baseline after implementation and record actual allocations/work;
 do not assert exact benchmark thresholds as tests.
 
+Completion: overlapping maps now merge into one owned lower copy; focused
+configuration tests pass. On Go 1.27.3, the documented 50-iteration, three-run
+workloads measured: depth/width 2/2 overlap 5,144 B and 131 allocations; 12/2
+overlap 19,952 B and 464; 40/2 overlap 72,962 B and 1,392; 4/16 overlap
+29,608 B and 765; 12/2 disjoint 32,496 B and 686. These are measurements,
+not performance guarantees.
+
 ## Stage 15: Centralize Trusted JSON Clone Ownership
 
 **Catalog:** 24. **Files:** `convert.go`, `internal/jsonvalue`, projection tests.