fix: preserve original .md URL from llms.txt for markdown-availability checks

[dacharyc]

Summary

• Fixes markdown-url-support: regression — llms.txt URL normalization breaks detection for sites using .html.md suffix #77: markdown-url-support regressed from 100% → 0% on sites whose llms.txt links use a .html.md suffix (Plaid, and any site emitting <page>.html.md siblings). normalizePageUrl rewrote the URL to its HTML form for sitemap dedup, then toMdUrls() regenerated candidates that missed the URL the site actually published.
• Carries the original .md/.mdx URL alongside the normalized URL through discovery as a sidecar map (originalMdUrls). markdown-url-support tries it first, then toMdUrls() candidates (with the existing mdFormPreference heuristic), then a parent-clean fallback (/foo/index.html.md → /foo.md) gated to URLs whose llms.txt original matched /index.html?\.md$.
• toMdUrls() itself is unchanged. The parent-clean candidate lives in markdown-url-support only, so other consumers (llms-txt-directive-md, llms-txt-links-markdown, tabbed-content-serialization, get-markdown-content) cannot regress to the prior false-positive class where unrelated sibling .md files passed validation.
• The mdFormPreference heuristic skips wins served via originalMdUrl so a run of .html.md sites doesn't skew the heuristic for other pages.

Test plan

• npm test — 1267/1267 passing
• npm run lint — clean
• New unit tests for originalMdUrls plumbing (.html.md, .mdx, plain .md, sitemap-only no-map, .md-form dedup, sampled-subset filtering, aggregate .txt walker propagation)
• New behavior tests in markdown-url-support covering issue markdown-url-support: regression — llms.txt URL normalization breaks detection for sites using .html.md suffix #77 plus sibling-page trap, no-leak guards (sitemap-only / plain .md / .mdx / /page.html.md no-index), parent-clean fallback when /index.html.md 404s, bounded request count, mdFormPreference skew guard, and pageResults.mdUrl reflects the served URL
• Fix-B isolation guardrail tests in llms-txt-directive-md and llms-txt-links-markdown confirming the parent-clean candidate cannot leak via toMdUrls()
• Smoke test against https://plaid.com/docs: markdown-url-support 0/155 → 10/10 (100%); every sample served via the original index.html.md URL