feat: add COBS framing module

Imports CobsDecoder and cobs_encode_response from firmware's
src/host/framing.rs, adjusted so the defmt::warn on encode overflow is
gated on the crate's defmt feature rather than an unconditional import.

The route_command helper stays in firmware — it depends on embassy
channels, which have no business in a transport-agnostic crate.

src/lib.rs

@@ -5,6 +5,8 @@
 
 #![no_std]
 
+pub mod framing;
+
 use heapless::Vec;
 
 /// Maximum LoRa payload size in bytes.