Avoid redundant decoded graph clones

internal/framework/evidencecontext/build.go

@@ -84,11 +84,11 @@ func Build(request BuildRequest) (Document, error) {
 		}
 		document.Contexts = append(document.Contexts, context)
 	}
-	canonical, err := canonicalize(document)
+	canonical, err := canonicalizeOwned(document)
 	if err != nil {
 		return Document{}, fmt.Errorf("validate evidence context: %w", err)
 	}
-	return clone(canonical)
+	return canonical, nil
 }
 
 type evidenceKey struct {

internal/framework/evidencecontext/codec.go

@@ -93,11 +93,11 @@ func (c *Codec) Decode(content []byte) (Document, error) {
 	if err := decoder.Decode(&trailing); err != io.EOF {
 		return Document{}, fmt.Errorf("decode evidence context: multiple JSON values")
 	}
-	canonical, err := canonicalize(value)
+	canonical, err := canonicalizeOwned(value)
 	if err != nil {
 		return Document{}, fmt.Errorf("decode evidence context: %w", err)
 	}
-	return clone(canonical)
+	return canonical, nil
 }
 
 func (c *Codec) schemaBytes() ([]byte, error) {
@@ -171,7 +171,10 @@ func canonicalize(value Document) (Document, error) {
 	if err != nil {
 		return Document{}, err
 	}
-	value = owned
+	return canonicalizeOwned(owned)
+}
+
+func canonicalizeOwned(value Document) (Document, error) {
 	if err := requireIdentity("source_id", value.SourceID); err != nil {
 		return Document{}, err
 	}
@@ -230,10 +233,7 @@ func canonicalizeContext(sourceID string, selected map[string]struct{}, seenUnit
 	}
 	positions := make(map[int]int, len(value.Units))
 	for unitIndex := range value.Units {
-		unit, err := cloneSourceUnit(value.Units[unitIndex])
-		if err != nil {
-			return Context{}, fmt.Errorf("%s.units[%d]: %w", prefix, unitIndex, err)
-		}
+		unit := value.Units[unitIndex]
 		if unit.ID <= 0 || strings.TrimSpace(unit.Kind) == "" || strings.TrimSpace(unit.Text) == "" {
 			return Context{}, fmt.Errorf("%s.units[%d] has invalid required fields", prefix, unitIndex)
 		}
@@ -251,7 +251,6 @@ func canonicalizeContext(sourceID string, selected map[string]struct{}, seenUnit
 		}
 		positions[unit.ID] = unitIndex
 		seenUnits[unit.ID] = struct{}{}
-		value.Units[unitIndex] = unit
 	}
 	if value.ContextRef.StartUnitID != value.Units[0].ID || value.ContextRef.EndUnitID != value.Units[len(value.Units)-1].ID {
 		return Context{}, fmt.Errorf("%s.context_ref must identify the first and last units", prefix)

internal/framework/evidencecontext/evidencecontext_test.go

@@ -180,12 +180,12 @@ func TestCodecRoundTripsCompactFixtureAndOwnsDecodedValues(t *testing.T) {
 		t.Fatalf("fixture does not use canonical encoding\nwant: %s\n got: %s", fixture, encoded)
 	}
 	value.Contexts[0].Units[0].Text = "changed"
-	decoded, err := codec.Decode(fixture)
+	decoded, err := codec.Decode(encoded)
 	if err != nil {
 		t.Fatal(err)
 	}
 	if decoded.Contexts[0].Units[0].Text != "The party meets Rowan." {
-		t.Fatal("Decode() reused mutable document storage")
+		t.Fatal("Encode() retained mutable document storage")
 	}
 	built, err := Build(BuildRequest{Source: testDocument(t), WindowUnits: 1, SelectedLanes: []string{"npcs"}, LaneEvidence: []LaneEvidence{{LaneID: "npcs", SourceRefs: []source.SourceRef{ref(3, 3)}}}})
 	if err != nil {