refactor(vtt-thumbnail): [PKG-VTT-THUMB-04] preserve legacy types and expose async runtime entry

packages/artplayer-plugin-vtt-thumbnail/README.md

@@ -6,6 +6,30 @@ Vtt thumbnail plugin for ArtPlayer
 
 [https://artplayer.org](https://artplayer.org/?libs=./uncompiled/artplayer-plugin-vtt-thumbnail/index.js&example=vtt.thumbnail)
 
+## TypeScript usage
+
+The registration function is asynchronous. The optional runtime entry provides its accurate
+Promise type and uses the same implementation as the default entry:
+
+```ts
+import Artplayer from 'artplayer'
+import vttThumbnail from 'artplayer-plugin-vtt-thumbnail/runtime'
+
+const art = new Artplayer({ container: '#player', url: '/video.mp4' })
+const result = await vttThumbnail({ vtt: '/thumbnails/index.vtt' })(art)
+console.log(result.name)
+```
+
+You can also pass `vttThumbnail({ vtt: '/thumbnails/index.vtt' })` in the player's `plugins`
+array. Existing default and `/legacy` imports remain available. Their declarations preserve
+the historical synchronous result type for compatibility, although registration has always
+returned a Promise at runtime. Reading `result.name` requires awaiting registration; changing
+to `/runtime` opts into that accurate type without changing runtime behavior. The option
+object remains required, and its `vtt` and `style` fields remain optional.
+
+See [ARCHITECTURE.md](ARCHITECTURE.md) for module ownership, testing commands and remaining
+compatibility validation.
+
 ## License
 
 MIT © Harvey Zhao