v10.19.0 doctor verifies content/vector coherence

Added

• Doctor check: every active memory's embed-text hash is re-derived from
  current content and compared to embed_meta.text_hash (recorded at
  embed time since v10.6 "so staleness is detectable later"; this is the
  later). Catches content mutated without a re-embed: direct SQL writes
  bypassing the API, or a write-path bug, both previously invisible to
  retrieval and to every other check. doctor --migrate re-embeds
  flagged rows. Near-total mismatch on stores of 20+ checkable rows is
  reported as an embed-text format change across versions rather than
  row corruption; rows predating hash tracking are skipped.