Migrate NPC registry durable contract

internal/modules/dnd/npcs/identity/identity_test.go

@@ -1,6 +1,7 @@
 package identity
 
 import (
+	"reflect"
 	"strings"
 	"sync"
 	"testing"
@@ -40,9 +41,16 @@ func TestNormalizeDisplayOnlyChangesWhitespace(t *testing.T) {
 
 func TestDeriveIDAndIDSyntax(t *testing.T) {
 	got := DeriveID("  Mira\u2003Thorn ")
+	const want = "npc:sha256:35ba5f679aee69e07ae3bd65c44278f29539d5dc9bb5225db1c0060555b23221"
+	if Policy != "dnd.npc_registry.identity.v1" || IdentityPolicy != Policy {
+		t.Fatalf("identity policy = %q/%q", Policy, IdentityPolicy)
+	}
 	if len(got) != len("npc:sha256:")+64 || !strings.HasPrefix(got, "npc:sha256:") || !IsValidID(got) {
 		t.Fatalf("DeriveID() = %q, want exact NPC ID syntax", got)
 	}
+	if got != want {
+		t.Fatalf("DeriveID() = %q, want SHA-256 of compact JSON [policy, comparison_name]", got)
+	}
 	if got != DeriveID("Mira Thorn") || got != IDFor("Mira Thorn") {
 		t.Fatal("DeriveID() is not deterministic across equivalent names")
 	}
@@ -80,7 +88,11 @@ func TestValidateRegistryReportsIdentityCollisionCategories(t *testing.T) {
 		{ID: validID, Name: "Mira Thorn"},
 		{ID: validID, Name: "Mira Thorn"},
 	}
+	before := append([]dnd.NPC(nil), npcs...)
 	issues := ValidateRegistry(npcs)
+	if !reflect.DeepEqual(npcs, before) {
+		t.Fatal("ValidateRegistry() mutated its input")
+	}
 	want := map[IssueCode]bool{
 		IssueDuplicateCanonical: false,
 		IssueDuplicateID:        false,