VSTD 1.2.0: add versioned documentation previews and external-link auditing

[TimeLordRaps]

Coordinate

VSTD 1.2.0 release-candidate public documentation infrastructure. Preserve normative ownership under
standard/; rendered pages must not become a competing specification.

Target

• Build and retain the GitHub Pages candidate as a pull-request artifact so reviewers can
  inspect presentation changes before merge.
• Distinguish released, unreleased-candidate, and historical documentation coordinates.
• Preserve stable canonical URLs for schemas and maintained guides.
• Add a scheduled external-link audit with retry and an explicit allowlist; begin
  non-blocking because third-party availability is not controlled by this repository.
• Retain accessibility and mobile-overflow checks on every change.

Exit criteria

• Every documentation pull request exposes a reviewable Pages artifact.
• A reader can identify the documentation's release coordinate without inference.
• Generated and normative sources have one declared ownership path.
• External failures are visible without making transient third-party downtime silently
  block unrelated work.

[TimeLordRaps]

Implemented in stacked pull request #27 at d941df4.

Evidence:

• Pull requests build and retain the exact GITHUB_SHA Pages candidate as pages-preview-${{ github.sha }}.
• The site emits documentation-coordinate.json with version, release state, source reference, canonical base URL, and normative source ownership.
• Candidate, historical, and released coordinates are explicitly distinguished; standard/ remains normative.
• A scheduled/manual external-link audit now retries, records JSON, uses an exact reasoned allowlist, and remains outside the pull-request gate.
• Presentation, accessibility/mobile-overflow, Pages assembly, and hosted presentation checks passed; run 33038825743 retained the Pages preview artifact.

The issue remains open for review and will be resolved by landing #27.