Adds `pseudonymize(actorId)` in core-audit — SHA-256 of salt+":"+actorId
truncated to 16 hex chars, prefixed "erased-". Salt from
AUDIT_PSEUDONYM_SALT env (fallback dev label). 6 unit tests: deterministic,
salt-change-differs, fallback-no-throw.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>