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.

AGENTS.md

@@ -15,6 +15,9 @@
 | `bun run typecheck` | `tsc --noEmit` - must be clean before finishing any change          |
 | `bun test src`      | Unit tests (`src/**/*.test.ts`)                                     |
 | `bun run compile`   | Clean build into `dist/` + post-compile step                        |
+| `bun run dev`       | Start the playground in `dev/` at http://localhost:7010             |
+| `bun run dev:types` | `tsc --watch`, for consumers working against `dist/`                |
+| `bun run dev:typecheck` | Typecheck the playground (`dev/tsconfig.json`)                  |
 | `./publish.sh "msg"`| Typecheck → test → compile → `git commit/push` → `bun publish`      |
 
 `publish.sh` pushes and publishes - only run it when explicitly asked. Bump `version` in `package.json` before publishing.
@@ -37,6 +40,7 @@
 | `src/(partials)/`, `src/(functions)/`, `src/(utils)/` | Internal helpers, not meant for direct import                  |
 | `src/types.ts`                            | Shared types/constants                                                   |
 | `scripts/post-compile.ts`                 | Copies `base.css`, adds `.js` to relative imports, prepends `"use client"` |
+| `dev/`                                    | Local bunext playground, own `node_modules`/lockfile; never published. `dev/twui -> ../src` drives HMR for library edits |
 
 ## Public API Rules (MANDATORY)