test: guard the public feature graph

[jderochervlk]

Tracking issue: #342

Stack context

Part 31 and final PR of the Option 5 folder-feature stack.

• Previous: refactor: clean up shared feature helpers #340
• Next: none; this closes the implementation stack

Review this PR as the incremental diff against #340.

Summary

• add npm run check:features to validate the exact 27 supported public features
• require one unqualified owning folder feature per public feature
• validate unique source features, source directories, and flat public module files
• compile every qualified WebAPI.* feature independently
• run the feature guardrail in CI
• generate API documentation directly from the rescript.json public module lists
• replace legacy nested feature/interface docs with flat interface pages
• document the distinction between consumer feature bundles and flat module paths

Guardrails

The new check fails when:

• the supported 27-feature list changes unintentionally
• a source folder uses a qualified public feature name
• a public feature lacks exactly one owning internal folder feature
• a public module is duplicated or its source file is missing
• any supported feature has an incomplete compiler dependency closure

Documentation update

The old documentation pipeline still read the pre-Option-5 migration taxonomy and expected nested interface routes. API pages and navigation now follow the actual flat public surface, producing one page per public interface module.

Temporary state

None from the Option 5 migration remains. Future feature changes should update the explicit contract and pass the per-feature compiler checks in this PR.

Review focus

• feature-contract assertions
• per-feature build behavior and CI placement
• flat API documentation discovery and navigation
• consumer feature documentation

Verification

• npm run check:features (all 27 supported features)
• npm run build
• npm test
• npm run build:docs (303 static pages)
• npm run format:check
• git diff --check

[jderochervlk]

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