refactor: consume donglora-protocol as a path dep

Pulls the wire protocol types and COBS framing out of firmware and into
the sibling ../protocol crate so client-rs, mux-rs, and bridge can share
them. Firmware's src/protocol.rs is now a thin re-export shim preserving
every crate::protocol::\* call site; src/host/framing.rs drops the
in-tree CobsDecoder / cobs_encode_response and re-exports the codec,
keeping only route_command (which needs embassy channels).

All seven board features still build clean, cargo fmt clean, cargo test
green.

Cargo.toml

@@ -29,6 +29,7 @@ embassy-usb = { version = "0.6", features = ["defmt"] }
 embassy-usb-driver = "0.2"
 ucobs = { version = "0.3", features = ["legacy-msrv"] }
 heapless = "0.9"
+donglora-protocol = { path = "../protocol" }
 lora-phy = "3.0"
 portable-atomic = { version = "1", optional = true, default-features = false, features = ["critical-section"] }
 static_cell = "2.1"