Add protobuf COM wrapper

docs/host_com_rebuild.md

@@ -82,6 +82,31 @@ Implemented behavior:
   - `Ack`
   - `Error`
 
+### Node 3: protobuf thin wrapper
+
+Files added in this step:
+
+- `KeyBorad/KeyBorad/COM/Com_PbCodec.h`
+- `KeyBorad/KeyBorad/COM/Com_PbCodec.cpp`
+
+Design notes:
+
+- stop expanding hand-written typed payload logic
+- let protobuf generated code own message encode/decode
+- keep only a thin Qt-friendly wrapper
+
+Implemented behavior:
+
+- serialize a full `CdcFrame`
+- parse and validate a full `CdcFrame`
+- build `HelloReq` using generated protobuf types
+- parse `HelloRsp` using generated protobuf types
+
+Current limit:
+
+- not wired into the project yet
+- protobuf runtime include and link setup is still pending
+
 ## COM done criteria
 
 For the current project stage, COM is considered done when: