fix(landing): fall back to English docs links for untranslated locale pages

[777genius]

What

useDocsLinks (landing app) built locale-specific docs URLs by blindly
regex-replacing the locale segment (/en/ -> /ru/ etc.) without checking
whether the target-locale page actually has real translated content. Per the
D3 locale disposition registry (website/tools/quality/locale-dispositions.json),
52 of 244 pages are english-fallback (no real translation yet) - notably the
whole use/ and build/ sections for non-English locales. Landing links into
those sections would have silently pointed non-English users at pages that
only exist in English, guessed as if they were localized.

Fix

• landing/scripts/generate-docs-locale-availability.mjs: build-time generator
  that derives a minimal per-locale "real content" manifest from the docs
  disposition registry (mirrors the existing build-pages-artifact.mjs
  cross-package script convention). Output is committed
  (landing/data/docsLocaleAvailability.generated.json), consistent with this
  repo's existing pattern of committing generated artifacts under
  website/generated/**.
• landing/utils/docsLinks.ts: new pure, framework-free helper
  (replaceDocsLocale/resolveDocsLocale/hasRealDocsContent) that consults
  the manifest and falls back to the English URL explicitly when the target
  locale has no real content for that page.
• landing/composables/useDocsLinks.ts: now passes the doc-relative path for
  each link (guide/quickstart, reference/support-boundary,
  guide/build-custom-plugin-logic, and the docs root) through the new helper
  instead of the old blind regex swap.
• landing/tests/docsLinks.test.ts: covers both the happy path (real content
  keeps the requested locale) and the fallback path (no real content ->
  English), plus the untouched-URL and unknown-locale-segment cases.

Scope note

Stacked on #178 (base docs/authoring-d4a-switcher-fix, same D2b/D3/generated
checkpoint chain) so this stays a small, independently reviewable slice ahead
of the larger Phase6 D4 landing implementation (Use/Build frontdoor,
header/footer, 5-locale SEO parity), which is tracked separately.

Verification

• node --test --experimental-strip-types tests/docsLinks.test.ts (pinned
  Node 22.21.1): 6/6 pass.
• Full pnpm run test:registry: 23/24 pass; the one pre-existing failure
  (registry.test.ts) is an unrelated missing local fixture
  (public/registry/schemas/1/latest.json, produced by the
  agentplugins-registry-mirror step), reproduces identically on the
  unmodified base branch, not something this PR touches or regresses.
• Isolated tsc --noEmit on the new utils/docsLinks.ts: clean.
• Currently the 4 existing links all resolve to historical-snapshot content
  in every locale, so there is no visible behavior change today - this closes
  the gap before the D4 landing PR adds new use//build/ links that do hit
  the fallback case.

No merge/squash from this session - opening for review only, per repo policy.

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