* Replace platform-specific optionalDependencies with bun devDependency
Fixes#222. The bun meta-package handles platform detection internally.
Rollup resolves its own platform binaries transitively. Removes ~400MB
of bloat for SDK consumers. Simplifies setup-bun.mjs accordingly.
* Remove setup-bun.mjs, simplify prepare script
No longer needed since the bun npm package handles binary
installation and PATH registration automatically.