odoc-interactive-extension: consolidate dual-compiler builds, per-package findlib, cross-jsoo compat

- Consolidate js_top_worker and odoc dual-compiler stanzas into single
library stanzas with cppo rules generating impl.ml from impl.cppo.ml
- Per-package findlib_index.json with relative universe paths (../dep)
and implicit stdlib dependency injection
- Add find_stdlib_dcs to Impl.S interface for stdlib CMI lookup via
findlib metadata instead of hardcoded URLs
- Replace jsoo Json.output/Json.unsafe_input with plain JSON.stringify/
JSON.parse for cross-jsoo-version compatibility (6.0.1+ox vs 6.2.0)
- Cross-origin worker support: set __global_rel_url in blob worker,
skip URL rewriting for absolute http(s) URLs
- Fix odoc doc comments and ocamlformat-ignore for cppo files
- Add demo docs, helper scripts, and x-ocaml package-lock.json

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>