Bun Module Implementation

- Automatic bun2nix CLI in devshell (converts bun.lock -> bun.nix)
- Generated package.json with postinstall script that runs bun2nix
- Hermetic app packaging via bun2nix.writeBunApplication
- run-<app> and test-<app> wrapper scripts
- Health checks for bun, bun2nix, and lockfile presence

- bun.nix lockfile (auto-generated via postinstall or: bun2nix)
- package.json in the app or root directory

Generated files (package.json) are created via stackpanel.files system
and materialized on shell entry. package.json uses type="json" for
deep-merge support so other modules can contribute to the same file.

stackpanel.apps.my-app = {
  path = "apps/web";
  bun = {
    enable = true;
    buildPhase = "bun run build";
    startScript = "bun run start";
    generateFiles = true;  # Generates package.json with bun2nix postinstall
  };
};