perf(theme): incremental theme registry rescan (skip unchanged files)

[ZhuchkaTriplesix]

Parent

Part of #414

Problem

Folder watcher debounce → loadAvailableThemes → full loadThemeDefinitions + LocalExtensionRegistry.reload() + read/parse every theme JSON + FNV hash of full content. Parsed theme LRU exists for loadTheme, not for definition scanning.

Evidence

• lib/core/theme/theme_registry_service.dart
• lib/core/theme/theme_folder_watcher.dart
• lib/core/theme/theme_controller.dart

Acceptance

• Unchanged files (mtime / contentHash) are skipped on refresh
• Theme-only watcher events do not reload the entire extension registry when unnecessary
• Cold full scan still works; hot path is incremental

Suggested fix

Incremental scan by mtime/contentHash; keep definition index; avoid full extension reload for theme-only events.

[ZhuchkaTriplesix]

Shipped on dev via #443 (Closes did not auto-close because PRs targeted dev).