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.
Copied src/protocol.rs from firmware verbatim, added crate-level #![no_std],
migrated the defmt::Format derive gate from cfg_attr(not(test), ...) to
cfg_attr(feature = "defmt", ...) so host-side (no-default-features) builds
compile cleanly. Tests unchanged; all 35 pass.
Firmware still owns the file for now — the re-export swap lands in a
follow-up firmware commit to keep each change standalone.
swaits