feat: add README corpus fidelity gate

[developer0hye]

Summary

• add the 50-repo README corpus validator, baselines, and CI workflow
• harden shared rendering for print fidelity with sanitization, TOC, print guards, and repo-root-relative asset support
• document and track the remaining advisory-only remote asset cases from the corpus

Why

• the PRD fidelity goal requires automated validation against real public README inputs, not manual inspection alone
• the corpus exposed real renderer and validator gaps that are now covered by regression tests
• the current validated state gives us a repeatable gate for future rendering changes

Key Changes

• add validation/readme_corpus.mjs, supporting libraries, pinned manifest, and committed baselines
• add corpus regression coverage for badge-heavy headings and repo-root-relative README assets
• fix local asset resolution for GitHub-style /path README image references
• tighten source snapshot normalization for badge-heavy headings and emphasis-fragmented words
• add the README corpus GitHub Actions workflow and supporting docs/reference notes

Verification

• cargo fmt --all
• node --test validation/readme_corpus.test.mjs
• node validation/readme_corpus.mjs verify-manifest --offline
• targeted corpus reruns for angular/angular and serde-rs/serde
• node validation/readme_corpus.mjs bless --tier all --force
  • 50 repos
  • 0 hard failures
  • 4 advisory-only warnings

Context

The remaining advisory-only warning cases are tracked separately in issue #1.

Related: #1