Render web-components reference in .md exports, and add check-md-exports audit script - #603

Description

Fixes

• Rename fern/products/browser-sdk/pages/v4/web-components/*.md → .mdx (27 files) and update v4.yml nav paths. These pages use <ParamField>/<CardGroup>, which Fern's .md-export path only compiles in .mdx files — every page in this section was exporting raw JSX to AI consumers while rendering fine as HTML. Verified on the preview: the exports now render every term properly.
• Add the MDX-required blank lines inside <Accordion> blocks in 5 v4 guides (house-style normalization per documentation-style.md; note the authentication guide's export corruption turned out to be a separate Fern exporter bug — fence imbalance in <EndpointRequestSnippet> output — reported upstream, not fixable from our MDX).
• Replace an authored 
 entity with a period in sw-audio-level's methods table.

New tooling:

• scripts/check-md-exports.js (yarn check-md-exports): audits the llms.txt tree + all .md exports for structural breakage. Tiered checks (error/warn/info), --cross-check mode verifying ParamField terms from sources against deployed exports (with --source-ref origin/main to read sources from the ref the site was built from, eliminating branch-drift noise), --base-url for previews, markdown/JSON reports, CI-ready exit codes. Zero dependencies, mirrors check-links.js conventions.
• scripts/check-md-exports.test.js (yarn test:scripts, 18 fixture tests on node:test): locks in the detection heuristics — the .md exports strip defined terms — llms.txt corpus is structurally broken #525 regression shape must trip, the legitimate older page styles must not, and the cross-check URL-mapping rules are pinned.
• scripts/README.md: documents all repo scripts, with a usage guide for the new audit.

Verification:

• fern check ✓; fern docs md check ✓ (2,806 MDX files valid on the rebased base); yarn test:scripts 18/18 ✓
• Full-corpus run against production: 3,156 pages, 34 errors, all triaged
• Preview run (--base-url <preview>/docs --product browser-sdk, 664 pages): 34 → 2 errors; both remaining are the upstream Fern snippet-fence bug
• --source-ref origin/main --product swml: 1,469 ParamField terms across 74 sources, zero missing from deployed exports (the 3 findings from the working-tree run were confirmed as branch drift, which this mode eliminates)

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Code cleanup / refactor

Related Issues

Closes #525.

Testing

• Added/updated unit tests
• Tested manually
• Tested with live SignalWire credentials (if applicable)

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the project's style guidelines
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)
• All existing tests pass

Additional Notes