📖✨：hand markdown-it the anchor and footnote plugins

[DerekNonGeneric]

markdown-it-anchor and markdown-it-footnote have both sat in devDependencies without ever reaching the markdown library — eleventy.config.mjs never touched it. So the syntax they exist to support has been rendered by whatever markdown-it does in their absence.

Footnotes were not merely absent, they were wrong

[^2] with a matching [^2]: definition is also perfectly good link reference syntax, so markdown-it happily rendered it as one. The published security page currently emits:

automated dependency management.<a href="https://www.mend.io/…">^2</a>

— the marker printed into the sentence as link text, rather than a superscript pointing at a footnotes section. After:

automated dependency management.<sup class="footnote-ref"><a href="#fn1" id="fnref1">[1]</a></sup>
…
<hr class="footnotes-sep"><section class="footnotes"><ol class="footnotes-list">

Headings had no ids

Which leaves any in-page link with nothing to find. Nothing published depends on one yet — but agent-forwarding.md links to #when-show-signature-says-no-signature twice and sets toc: true, so it would have shipped broken. It is draft: true, so this is latent rather than live. Verified the target now resolves in a drafts build.

Verification

• headings now carry ids (<h2 id="issue-tracker">, <h3 id="resolving-open-issues">, …)
• all ids are unique across all 27 pages — checked, since duplicates would be both a validity error and a navigation bug
• the W3C validator stays clean across all 27 pages, which matters because 🏗️🔧：validate every built page, not just the home page #1786 widened it to the whole site and this adds new markup
• nps build, nps test, nps format.all all pass

Deliberately not included

You asked me to look at what else could be wired up rather than deleted. Two findings, neither belonging in this PR:

The retext prose-linting pipeline has never run. Not the two unused plugins — the whole thing. remark-lint rules fire correctly, but nothing in naturalLanguage does: retext-repeated-words does not flag has has, retext-simplify does not flag utilize. The plugins are passed as module namespace objects rather than functions, so the bridge never attaches — silently, with no error. Forcing it surfaces the real failure:

TypeError: attacher.call is not a function
    at apply.remarkRetext (…/remark-retext/lib/index.js:88:35)

Unwrapping .default across the pipeline makes it work — and then reports 3,215 messages on existing content:

rule	messages
retext-spell	3034
retext-readability	78
retext-simplify	46
retext-repeated-words	22
retext-equality	22
retext-profanities	5
retext-passive	3

That is its own project, and worth deciding on first: retext-spell accounts for 94% of it while duplicating cspell, which already runs and passes. Likely it should go and the rest be turned on.

The remaining unwired packages are genuinely inert here, so no deletions in this PR: remark-github, remark-html, remark-heading-id, remark-hint, and remark-directive all transform or render markdown, but remark is only ever run as a linter (remark -f) whose output is discarded — wiring them would change nothing visible. remark-lint-code-block-style and remark-lint-definition-spacing are already applied via remark-preset-lint-markdown-style-guide@^4.0.0, so declaring them directly is redundant rather than useful. markdown-it-abbr, -sub, -sup, and -task-lists have no content using their syntax (the docs write <abbr> by hand).

Part of #254.

List of any relevant issue numbers: #254

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