resolveIdentity runs on every register (block + flap gates). It already returned on
device_id before any DB access; memoized the device_fingerprints statement (prepared
once, lazily) and documented the invariant. Test asserts a device_id-present resolve
prepares/runs ZERO device_fingerprints queries; a device_id-absent resolve does.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
