docs(authoring): prepare site consumer, historical routes, and locale corrections (D2b/D3)

[777genius]

Scope

Stacked on the combined D1/D2a dependency base (chore/authoring-doc-dependencies, itself merging #171 English use/build journeys and #175 CLI exporter). This PR adds the D2b/D3 preparation work on top:

• docs: prepare authoring site consumer and historical routes - site consumer wiring, generator, historical route preservation, immutable adapter pin at source SHA 801f846c.
• fix(docs): preserve historical pages and require reproducible adapter assembly
• fix(docs): restore pinned historical Claude event pages
• docs: prepare maintained locale fallbacks and preserve v1 content
• docs: compose locale preparation for sandbox validation
• fix(docs): preserve localized archives and verify locale navigation - resolves 4 correction findings (mandatory consumer mismatch/English-only sidebar, locale tests outside the mandatory graph, permanent fallback/archive classification, preservation depending on a private integration object).

Why this shape

801f846c and everything after it depend on both #171 and #175 (verified: git merge-base --is-ancestor true for both PR heads). Basing directly on either PR alone would either omit the other PR's prerequisite commits or re-include them as "new" diff noise. chore/authoring-doc-dependencies is a plain merge of the two PR heads (no independent content of its own) pushed as a shared dependency branch, following this repo's existing stacked-base convention (compare feat/authoring-private-npm-base, which is itself only a dependency branch and not a tracked PR).

Evidence already gathered

• Independent MEDIUM-effort review accepted this correction commit for bounded preparation; all four original findings resolved (site-consumer mismatch, locale test wiring, fallback/archive classification, Git-history preservation dependency). One non-blocking follow-up was raised about switcher browser-test coverage (viewport containment / full activation), which is addressed separately in a follow-on PR stacked on top of this one and the generated-baseline PR.
• Mandatory offline suite: 23/23 tests, 0 skipped, real adapter assembly (no overlay), locale checker 61 routes x 5 locales passing.
• VitePress production build completed cleanly (~86s), postbuild/output checks green, actual browser evidence collected without diagnostic overlays.
• All 192 historical archive pages independently verified byte-identical to their original frontmatter/body; 52 localized English-fallback pointers verified as honest pointers (no fabricated translations).

Explicitly not claimed

• Not a release-ready or main-mergeable state. Public docs main auto-deploys, so this must not land ahead of the D3/D4/D5 and platform release gates (see docs/AUTHORING_CAPABILITY_PRESERVATION.md and the implementation plan's release gates). Kept as a draft for that reason.
• Does not include Phase 6 D4 landing (Use/Build frontdoor, header/footer, useDocsLinks rewrite) - tracked separately.
• Does not include the generated CLI/API reference baseline - that is the next PR in this stack, kept isolated because its diff is machine-generated.

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