nx_tls13_mtls_finished.nx

nx_tls13_mtls_finished.nx -- R7: the mTLS-aware client Finished handler (the dual-snapshot key schedule). In mutual TLS the server receives the client's Finished AFTER the client Certificate + CertificateVerify, so the transcript at that point runs THROUGH ClientCertVerify. RFC 8446 §4.4.4 says the client Finished MAC is over that (current) transcript -- but §7.1 says the application traffic secrets are derived over the transcript THROUGH SERVER FINISHED. A plain recv_cf uses ONE snapshot for both => wrong app keys under mTLS. This handler takes th_app (the through-ServerFinished snapshot, captured by the run loop right after emit_sf) explicitly: it verifies the Finished MAC over the CURRENT transcript snapshot, and derives the app keys over th_app. Extracted from the run loop so this exact split is GATED (not just compile-checked): the gate proves the function uses th_app for app keys and the current transcript for the MAC, by passing two DIFFERENT values. license_tier: ORIGINAL expect_exit: 0