mcpmesh-codec: the family's ONE wire codec (mcpmesh spec §7.3) — compact JSON, UTF-8,
one frame per \n, 16 MiB cap.
Both ends of every wire share THIS implementation: the daemon side re-exports it as
mcpmesh_net::framing, and the no-iroh client side (kb, loc, the host shell) as
mcpmesh_local_api::codec — so the two ends can never drift apart. Deliberately tiny:
serde_json + tokio io traits only, NO iroh. Session POLICY (violation strikes,
synthesized error frames) stays with the session owners (mcpmesh-net); only the pure
codec lives here.