Release v1.2.2: reading and link validation patch

[ikelaiah]

Summary

Prepare DocSprout v1.2.2 as a compatibility-preserving patch release.

• Add a keyboard skip link, compact the phone header after scrolling, and measure progress against the reading article.
• Reject broken same-page heading fragments and links to missing or unpublished Markdown during check and build.
• Update version metadata, the historical-site manifest, workflow and example pins, changelog, migration guidance, and qualification evidence.

Verification

• 307 unit tests pass on the local Windows/Python 3.13 checkout.
• Ruff, docsprout check, and docsprout audit --strict pass.
• The 41-page documentation site builds.
• Wheel and sdist build and pass artifact inspection.
• Clean installs from both artifacts pass installed-package qualification.
• GitHub CI passes on the release branch (Linux, Windows, macOS, package, lint, and documentation jobs).
• Isolated Chrome review: 320, 390, 768, 1024, and 1440 px layouts; phone and desktop qualification pages; Light and Dark modes; skip link, search, outline, heading jump, compact header, and article progress. No horizontal overflow or console errors.
• Chrome accessibility tree exposes the expanded disclosure and labelled page outline; the current heading link has aria-current="location".

A live screen reader was unavailable in this environment, so speech output was not checked.

docsprout doctor reports only the expected missing v1.2.2 tag. The tag must be created on the merged commit, after this PR is green.