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.lock

@@ -495,6 +495,7 @@ dependencies = [
  "defmt 1.0.1",
  "defmt-rtt",
  "display-interface",
+ "donglora-protocol",
  "embassy-embedded-hal",
  "embassy-executor",
  "embassy-futures",
@@ -523,6 +524,15 @@ dependencies = [
  "ucobs",
 ]
 
+[[package]]
+name = "donglora-protocol"
+version = "0.1.0"
+dependencies = [
+ "defmt 1.0.1",
+ "heapless 0.9.2",
+ "ucobs",
+]
+
 [[package]]
 name = "either"
 version = "1.15.0"