Add candidate artifact codec decoding

internal/modules/dnd/codec/npcregistry/codec_test.go

@@ -112,6 +112,39 @@ func TestCodecCandidatePreservesInvalidTypedValues(t *testing.T) {
 	}
 }
 
+func TestCodecCandidateDecodeOwnsValues(t *testing.T) {
+	codec := New()
+	input := validList()
+	content, err := codec.EncodeCandidate(input)
+	if err != nil {
+		t.Fatal(err)
+	}
+	first, err := codec.DecodeCandidate(content)
+	if err != nil {
+		t.Fatal(err)
+	}
+	second, err := codec.DecodeCandidate(content)
+	if err != nil {
+		t.Fatal(err)
+	}
+	nameOffset := bytes.Index(content, []byte("Mira Thorn"))
+	if nameOffset < 0 {
+		t.Fatal("candidate JSON does not contain NPC name")
+	}
+	content[nameOffset] = 'X'
+	if first.NPCs[0].Name != "Mira Thorn" {
+		t.Fatal("DecodeCandidate() retained input bytes")
+	}
+	first.NPCs[0].Name = "changed"
+	first.NPCs[0].SourceRefs[0].SourceID = "changed"
+	if input.NPCs[0].Name != "Mira Thorn" || input.NPCs[0].SourceRefs[0].SourceID != "session-alpha" {
+		t.Fatal("DecodeCandidate() retained input values")
+	}
+	if second.NPCs[0].Name != "Mira Thorn" || second.NPCs[0].SourceRefs[0].SourceID != "session-alpha" {
+		t.Fatal("DecodeCandidate() returned aliased values")
+	}
+}
+
 func TestCodecRejectsEveryRequiredShapeBoundary(t *testing.T) {
 	base := validList().NPCs[0]
 	tests := []struct {