nx_turn_msgintegrity.nx

nx_turn_msgintegrity.nx -- TURN MESSAGE-INTEGRITY (RFC 5389 §15.4). Arc 4 Phase A.4 layer; split from nx_turn_relay.nx so the framing layer can be imported by x86_64 daemons without dragging in nx_hmac_sha1 (which uses the RISC-V-numbered nx_syscalls clan and would clash with nx_udp's nx_syscalls_x86_64 clan). See [[reference-syscall-module-duality-nx_syscalls-vs-syscalls]] for why this split is necessary. Daemons targeting x86_64 import nx_turn_relay (framing only); WASM-target smokes can import both since the JS shim resolves either clan. HMAC-SHA1 over the message, with key = SASLprep(password) for short-term creds. Long-term creds (MD5(user:realm:pw)) queued.