refactor: remove obsolete website code and check unused exports

[fhammerschmidt]

Obsolete compiler installations, scripts, components, and exports remained after the website and guide migrations. Remove the confirmed dead paths and make unused ReScript values and exports fail CI across all workspaces.

Fixes #1409. Part of #1421.

• Delete the unused compiler installation, CodeMirror 5 setup, scripts, shims, duplicate MDX components, unmounted scroll-lock context, and stdlib-toc.json.
• Trim unused URL/version helpers, Babel AST cases, warning-letter groups, guide output fields, and public interfaces. Keep live MDX mappings, compiler hover hints, internal decoders, and script dependencies such as jsdom.
• Remove four unused direct dependencies and their orphaned lockfile entries.
• Keep 44 necessary @live annotations: 42 in .resi interfaces and two in standalone .res modules. Remove duplicate implementation annotations and document annotating each entry point only once. Add yarn check:dead-code and a CI gate for unused values/exports; underscore bindings are intentional discards, while record-field and argument diagnostics remain advisory.

Branch: fix/1421-dead-code, based on website master (v12). No compiler repository changes.

Validation: strict ReScript compilation, dead-code check, a real temporary-unused-export failure probe, five diagnostic-check tests, 56 existing Node script tests, guide production build, and formatting pass. Docs production build and validation of all 188 example pages and relative links also pass. Removing each of the 44 remaining annotations individually produces an unused-value/export finding, confirming that all are necessary with the current analyzer. Strict compilation, formatting, and the dead-code gate pass locally. The homepage CSS prefetch test now waits for the client router and an observable React state update before focusing the prerendered link, preserving its initial absence, focus-prefetch, and blur-cleanup assertions. All six CI jobs pass on the latest commit; HomepageRouteCss passes all nine tests and the homepage suite passes all 40 tests.

Integration: keep this cleanup last among the #1421 PRs. Other API, routing, compiler-settings, and playground changes touch the same code and may supersede parts of this cleanup; rebase and drop those edits as the earlier PRs land. The branch is rebased onto current master, preserving the guide's shared favicon imports and the removal of its obsolete output directory. No static assets or screenshots were changed.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-09T23:25:05.929130Z	3cd3497	PR opened

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