Parallel interface to PurposeGuardProvider for runtime subject
resolution. Seven concrete resolvers, one per v1.0 purpose. Wired
through purposes.php via subject_resolver_class key.
EventRegistration uses PersonProvisioner (may create). Other purposes
resolve from existing context (portal token, production request, auth).
IncidentReport is the only purpose allowed to return null (anonymous-
allowed configurations); the others return concrete model types
(narrowed via PHP covariance) for caller convenience.
Refs: RFC-WS-6.md §3 (Q9)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>