feat(seo): per-page Open Graph/Twitter metadata + accurate, length-capped descriptions

[raifdmueller]

Closes #601

Implements findings F1–F3 (+F5) from the post-#598 SEO review.

Changes

1. Per-page social metadata (applyHead()): every pre-rendered page — 16 routes, home EN/DE, 161+155 anchor pages — now rewrites og:url, og:title, og:description, twitter:url/title/description to its own values (matching title/description/canonical). German pages get og:locale=de_DE with en_US alternate. Previously all subpages shipped the shell's home values, so sharing /anchor/mece rendered the generic home preview.
2. Accurate home description: the hard-coded "110+ semantic anchors" (real count: 161) is replaced by counts computed from anchors.json/contracts.json at build time — "161 semantic anchors and 19 semantic contracts …". The static shell texts (meta, og, twitter, WebSite JSON-LD) are now count-free so they cannot go stale again.
3. Normalized description lengths: all meta descriptions are capped at ~160 chars on a word boundary (capDescription()); anchor extracts under 50 chars get the anchor title prefixed ("Clean Architecture — Dependencies only point inward").
4. v0.6.1 + changelog.

Verification

• Build sweep over all 340 pre-rendered pages: every description is 40–160 chars (entity-decoded), zero too-short, zero truncation candidates.
• dist/anchor/mece/: own og:url/og:title/twitter:title; dist/de/about/: og:locale=de_DE + en_US alternate.
• Home EN/DE descriptions carry the computed counts (161/19).
• 102 unit tests, 35 E2E tests, ESLint + Prettier green.

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📥 Commits

Reviewing files that changed from the base of the PR and between d044765 and 5b2f39c.

📒 Files selected for processing (4)

• docs/changelog.adoc
• scripts/prerender-routes.js
• website/index.html
• website/src/main.js

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