Fix two tester findings:
1. Missing engine projection now fails closed: if v2Core is active but
CoreProjection(path) is missing, gate locally with reason
"missing_engine_projection". Mirrors T2's fail-closed posture.
Only skips enforcement when V2 core is entirely absent.
2. NVMe/TCP now gated alongside iSCSI: gateServing() calls both
targetServer.DisconnectVolume() and nvmeServer.RemoveVolume().
ungateServing() re-registers with both iSCSI and NVMe. A gated
volume is unreachable through all frontend paths.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
