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.

internal/configuration/targets.go

@@ -138,24 +138,33 @@ func overlayProviderMap(lower, upper map[string]any) (map[string]any, error) {
 	if merged == nil {
 		merged = make(map[string]any)
 	}
+	if err := overlayOwnedProviderMap(merged, upper); err != nil {
+		return nil, err
+	}
+	return merged, nil
+}
+
+func overlayOwnedProviderMap(merged, upper map[string]any) error {
 	for key, value := range upper {
 		if child, ok := value.(map[string]any); ok {
 			if existing, ok := merged[key].(map[string]any); ok {
-				combined, err := overlayProviderMap(existing, child)
-				if err != nil {
-					return nil, err
+				if existing == nil {
+					existing = make(map[string]any)
+					merged[key] = existing
+				}
+				if err := overlayOwnedProviderMap(existing, child); err != nil {
+					return err
 				}
-				merged[key] = combined
 				continue
 			}
 		}
 		copied, err := jsonvalue.Copy(value)
 		if err != nil {
-			return nil, err
+			return err
 		}
 		merged[key] = copied
 	}
-	return merged, nil
+	return nil
 }
 
 func (state *loaded) lookupProfile(ctx context.Context, id string) (domain.ExecutionProfile, error) {

internal/configuration/targets_test.go

@@ -42,6 +42,28 @@ func TestLoadIndexesIdentifiedTargets(t *testing.T) {
 	}
 }
 
+func TestOverlayProviderMapOwnsInputsAndReplacesValues(t *testing.T) {
+	lower := map[string]any{"nested": map[string]any{"keep": "lower", "replace": []any{"old"}, "null": "old"}, "disjoint": []any{"original"}}
+	upper := map[string]any{"nested": map[string]any{"replace": []any{"new"}, "null": nil, "added": map[string]any{"value": "upper"}}}
+	got, err := overlayProviderMap(lower, upper)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := map[string]any{"nested": map[string]any{"keep": "lower", "replace": []any{"new"}, "null": nil, "added": map[string]any{"value": "upper"}}, "disjoint": []any{"original"}}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("overlay = %#v", got)
+	}
+	lower["disjoint"].([]any)[0] = "changed"
+	upper["nested"].(map[string]any)["replace"].([]any)[0] = "changed"
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("overlay shares input storage: %#v", got)
+	}
+	got["nested"].(map[string]any)["added"].(map[string]any)["value"] = "changed"
+	if upper["nested"].(map[string]any)["added"].(map[string]any)["value"] != "upper" {
+		t.Fatal("result mutation reached upper")
+	}
+}
+
 func TestRequestCompletesLoadedPartialProfile(t *testing.T) {
 	state, err := load(mapping("partial profile", map[string]any{
 		"profiles": []any{map[string]any{"id": "partial"}},