fix: pin the guide compiler and validate lesson output

[fhammerschmidt]

The guide previously selected the newest stable compiler on the CDN during each build. Lessons were never compiled or checked against their expected output, and a failed versions download could publish the guide without a compiler.

Pin the guide to ReScript v12.3.1 and validate lessons during prerendering:

• Compile each starter and reference solution with the same published browser compiler, libraries, module system, and warnings as the learner's editor.
• Reuse the browser's source instrumentation and output transformation, load the matching CDN runtime imports, and require an exact matching checkpoint log line.
• Add optional build-only exercise.solutionCode, defaulting to the starter. The function lesson supplies a reference solution while retaining the original exercise for learners.
• Stop builds on download, compile, runtime, or output failures, with source-file diagnostics. Reference solutions are excluded from published lesson data.
• Report failed or stalled browser compiler loading; a stalled request displays a reload message after 15 seconds.
• Share compiler endpoint configuration and console serialization with fix: share guide and playground compiler helpers #1441, keeping both PRs independently buildable.
• Document compiler upgrades and the current validation contract: synchronous output, no browser-only APIs, and a one-second execution limit.

Targets website master (v12). No compiler repository changes.

Validation:

• Strict ReScript compilation and formatting pass.
• All 16 real compiler validation tests pass locally, including both published lessons, invalid starters and solutions, exact output mismatches, final-expression instrumentation, runtime imports and errors, runaway code, and CDN/version failures.
• Guide production build passes. A deliberately incorrect checkpoint fails the production build with its lesson filename.
• Generated HTML and loader data exclude reference solutions.
• All six GitHub Actions jobs pass after the compatibility update, including all 16 real compiler checks and 53 guide browser tests. Browser regressions cover failed and stalled compiler downloads.
• A temporary merge with fix: share guide and playground compiler helpers #1441 is conflict-free and passes strict compilation, all 97 Node tests, and the guide production build with lesson validation enabled.

Fixes #1405.
Part of #1421.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-09T21:49:16.617083Z	9145607	PR opened

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