feat: add JTW (js_top_worker) support for browser REPL artifacts
Enable ohc to produce JavaScript toplevel artifacts alongside
documentation, so every (package, version, universe) triple gets a
working browser-based OCaml REPL with that package pre-loaded.
Adds --with-jtw and --jtw-output flags to health-check and batch
commands. Creates jtw-tools layers (js_of_ocaml + js_top_worker per
OCaml version), per-package jtw layers (.cma.js, .cmi, META,
dynamic_cmis.json), and assembles output with compiler/, u/, and p/
directory structure.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>