feat(js_top_worker): remove ppxlib from worker binary, use lazy loading
ppxlib (~5.5MB of bytecode) was statically linked into every worker.js,
even though most users never use PPX rewriters. Replace the custom
JsooTopPpx module with the standard OCaml toplevel mechanism:
Ast_mapper.register + Toploop.preprocess_phrase. PPX packages are now
loaded on-demand via #require, with ppxlib_register bridging ppxlib
into the Ast_mapper pipeline.
Worker size: 67MB → 56MB (11MB / 16% reduction).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>