build: migrate bundler from tsup to tsdown

Replace tsup with tsdown (rolldown-powered). Config ports the two ESM
entries (index, mux), dts, and clean. The stylesheet now ships via
tsdown's native copy (src/styles/kino.css -> dist/styles.css) instead
of an onSuccess/cp shell step, so the build script is just `tsdown`.

fixedExtension: false keeps .js/.d.ts output to match the existing
package.json exports. react/react-dom (peer) and @mux/mux-video (dep)
remain external by default. Build, typecheck, lint, and tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: karngyan

package.json

@@ -40,7 +40,7 @@
     "./styles.css": "./dist/styles.css"
   },
   "scripts": {
-    "build": "tsup && cp src/styles/kino.css dist/styles.css",
+    "build": "tsdown",
     "dev": "vite --config demo/vite.config.ts",
     "test": "vitest run",
     "test:watch": "vitest",
@@ -76,7 +76,7 @@
     "prettier": "^3.3.0",
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
-    "tsup": "^8.3.0",
+    "tsdown": "^0.22.3",
     "typescript": "^5.7.0",
     "typescript-eslint": "^8.0.0",
     "vite": "^6.0.0",