Tailscale's TS2021 protocol layers HTTP/2 over an encrypted Noise IK
channel reached via HTTP CONNECT-style upgrade. Add the lower half:
- noise/handshake: hand-rolled Noise_IK_25519_ChaChaPoly_BLAKE2s
initiator with HKDF + ChaCha20-Poly1305 (no snow dependency)
- noise/framing: 3-byte frame codec (1-byte type + 2-byte BE length)
- noise/stream: NoiseStream implementing AsyncRead + AsyncWrite over
the framed channel so the h2 crate can sit on top
