TestAudit_OwnerSpoof: a body declaring a foreign owner endpoint or signing key
is 403; a self-owned create is 201.
TestAudit_NonceCachePoisonPreAuth: an unregistered identity's repeated nonce
still fails 'not authorized' (never 'replayed'), proving it was not cached, while
an authorized identity's replay is still rejected.
Nonce cache unit tests: prune-after-TTL and cap-bounded memory.
