The Normative Mappings page overlapped both neighbours, so split and remove it:

- External-vocabulary content (fidelity tiers, the mapping-targets table,
  where the mappings live) moves into the generated External Vocabularies
  page via prep-site.mjs, under a new "Normative mappings" section.
- The auditable-trail content (sources, the defensible fork, traceability,
  validation) moves into Governance, consolidating the former
  "Defensibility: the ledgers" section and fixing a stale claim that terms
  carry in-band dcterms:source to BSP (removed by the ledger convention).
- Delete app/docs/mappings/page.mdx; drop it from nav (_meta) and the home
  cards; repoint every /docs/mappings link to /docs/external-vocabularies
  (or the in-page #normative-mappings anchor).

Rebuilt: no /docs/mappings references remain, out/docs/mappings gone,
validate-vocab.py green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>