refactor(jassub): [PKG-JASSUB-04] migrate owned TypeScript and preserve public types

packages/artplayer-plugin-jassub/src/jassub.es.d.ts

@@ -0,0 +1,12 @@
+import type { RuntimeInstance, RuntimeOption } from '../types/runtime-api.js'
+
+interface VendorInstance extends RuntimeInstance {
+  _destroyed?: boolean
+  // Minimal write contract: the DOM setter coerces the historical numeric 20.
+  _canvasParent?: { style: { zIndex: string | number } }
+}
+
+declare const JASSUB: {
+  new (option: RuntimeOption): VendorInstance
+}
+export default JASSUB