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/native.go

@@ -6642,81 +6642,6 @@ func marshalOrderedLockfile(keys []string, lockData map[string]int, formatting j
 	return buffer.Bytes(), nil
 }
 
-func loadLegacyTLK(root string) (*legacyTLKData, error) {
-	info, err := os.Stat(root)
-	if err != nil {
-		if os.IsNotExist(err) {
-			return nil, nil
-		}
-		return nil, err
-	}
-	if !info.IsDir() {
-		return nil, fmt.Errorf("legacy tlk root must be a directory: %s", root)
-	}
-
-	result := &legacyTLKData{
-		Language: "en",
-		Entries:  map[string]tlkEntryData{},
-		Lock:     map[string]int{},
-	}
-
-	lockPath := filepath.Join(root, "lock.json")
-	lockData, err := loadLockfile(lockPath)
-	if err != nil && !os.IsNotExist(err) {
-		return nil, err
-	}
-	for key, id := range lockData {
-		result.Lock[key] = id
-	}
-
-	basePath := filepath.Join(root, "base.json")
-	if _, err := os.Stat(basePath); err == nil {
-		base, err := loadJSONObject(basePath)
-		if err != nil {
-			return nil, err
-		}
-		if language, ok := base["language"].(string); ok && language != "" {
-			result.Language = language
-		}
-		if entries, ok := base["entries"].(map[string]any); ok {
-			normalized, err := normalizeLegacyTLKEntries(entries)
-			if err != nil {
-				return nil, err
-			}
-			for key, entry := range normalized {
-				result.Entries[key] = entry
-			}
-		}
-	}
-
-	modulesDir := filepath.Join(root, "modules")
-	modulePaths, err := collectModulePaths(modulesDir)
-	if err != nil {
-		if os.IsNotExist(err) {
-			return result, nil
-		}
-		return nil, err
-	}
-	for _, path := range modulePaths {
-		obj, err := loadJSONObject(path)
-		if err != nil {
-			return nil, err
-		}
-		entries, ok := obj["entries"].(map[string]any)
-		if !ok {
-			continue
-		}
-		normalized, err := normalizeLegacyTLKEntries(entries)
-		if err != nil {
-			return nil, fmt.Errorf("%s: %w", path, err)
-		}
-		for key, entry := range normalized {
-			result.Entries[key] = entry
-		}
-	}
-	return result, nil
-}
-
 func normalizeLegacyTLKEntries(entries map[string]any) (map[string]tlkEntryData, error) {
 	normalized := map[string]tlkEntryData{}
 	for _, key := range sortedKeys(entries) {