[reports] Validate published HTML with proof-html

[coisa]

Related Issue

Closes #178

Motivation / Context

• The reports workflow already verifies that published GitHub Pages URLs respond after deployment.
• It did not validate the published HTML documents themselves, so malformed markup or broken internal links could still slip through after deploy.

Changes

• Add proof-html validation jobs after the existing main and preview deployment health checks.
• Validate the published gh-pages content for main reports, excluding historical PR previews from the main-site scan.
• Validate the published PR preview directory before posting preview links back to the pull request.
• Include HTML validation results in the reports workflow summary.
• Document the new reports validation behavior and add a changelog entry.

Verification

• composer dev-tools
• Focused command(s): composer dev-tools code-style -- --fix --json
• Focused command(s): composer dev-tools changelog:check
• Manual verification: ruby -e 'require "yaml"; %w[.github/workflows/reports.yml resources/github-actions/reports.yml].each { |f| YAML.load_file(f); puts "#{f}: ok" }'
• Manual verification: git diff --check

Documentation / Generated Output

• README updated
• docs/ updated
• Generated or synchronized output reviewed

Changelog

• Added a notable CHANGELOG.md entry

Reviewer Notes

• The new proof-html jobs intentionally run after the existing deployment verification jobs. The health checks continue to absorb GitHub Pages propagation concerns, while proof-html validates the checked-out published content from gh-pages.
• External link checking is disabled so the workflow does not fail on third-party transport instability; the validation focuses on the published HTML and internal link graph.
• I did not run the full composer dev-tools locally because this branch only changes workflows/docs/changelog and the repository pre-commit composer_script has recently timed out at 120 seconds while running that full suite. The focused fix/check commands above passed before commit.

[coisa]

Closing this PR after validating the experiment in Actions. proved too expensive for the generated reports surface and produced noisy failures inside generated assets we do not directly control. The useful CI follow-up discovered here will be split into a dedicated PR for the wiki pointer required-check issue instead.