topdata: remove completed legacy-migration cluster

The one-time legacy normalize migration finished; NormalizeProject had no
production callers. Delete migrate.go, the 26 *_migrate.go files, the
importLegacyItempropsRegistry cluster, loadLegacyTLK, and the 48 test
blocks that only exercised them. mergeLegacyTLK moves to tlk_native.go
because the live TLK compiler still uses it for base_dialog.json.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/topdata/tlk_native.go

@@ -727,3 +727,36 @@ func sortedKeys[M ~map[string]V, V any](input M) []string {
 func almostEqualFloat32(a, b float32) bool {
 	return math.Abs(float64(a-b)) < 0.0001
 }
+func mergeLegacyTLK(base, override *legacyTLKData) *legacyTLKData {
+	if base == nil && override == nil {
+		return nil
+	}
+	merged := &legacyTLKData{
+		Language: "en",
+		Entries:  map[string]tlkEntryData{},
+		Lock:     map[string]int{},
+	}
+	if base != nil {
+		if base.Language != "" {
+			merged.Language = base.Language
+		}
+		for key, entry := range base.Entries {
+			merged.Entries[key] = entry
+		}
+		for key, id := range base.Lock {
+			merged.Lock[key] = id
+		}
+	}
+	if override != nil {
+		if override.Language != "" {
+			merged.Language = override.Language
+		}
+		for key, entry := range override.Entries {
+			merged.Entries[key] = entry
+		}
+		for key, id := range override.Lock {
+			merged.Lock[key] = id
+		}
+	}
+	return merged
+}