Add bunext playground for local component development

Runs at http://localhost:7010 via bun run dev. The playground imports the library from src through a committed symlink and tsconfig path alias, so component edits hot-reload without a build step. It has its own install root and is excluded from the published package.

package.json

@@ -38,7 +38,9 @@
         }
     },
     "scripts": {
-        "dev": "tsc --watch",
+        "dev": "bun run --cwd dev dev",
+        "dev:types": "tsc --watch",
+        "dev:typecheck": "bun run --cwd dev typecheck",
         "compile": "rm -rf dist tsconfig.tsbuildinfo && tsc -p tsconfig.json && bun scripts/post-compile.ts",
         "typecheck": "tsc -p tsconfig.json --noEmit",
         "test": "bun test src",