Add COM protocol entry

docs/host_com_rebuild.md

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+# Host COM Rebuild
+
+## Goal
+
+Keep the protocol layer small and explicit.
+
+The COM layer should:
+
+- define packet-facing helpers
+- own frame parsing and packet building
+- avoid transport concerns
+- avoid UI and session concerns
+
+## Completed Nodes
+
+### Node 1: legacy protocol baseline
+
+Files already present before this step:
+
+- `KeyBorad/KeyBorad/COM/Com_Cdc.h`
+- `KeyBorad/KeyBorad/COM/Com_CdcEncode.h`
+- `KeyBorad/KeyBorad/COM/Com_CdcEncode.cpp`
+- `KeyBorad/KeyBorad/COM/Com_CdcDecode.h`
+- `KeyBorad/KeyBorad/COM/Com_CdcDecode.cpp`
+
+Design notes:
+
+- these files preserve the old packet format
+- they already encode and decode complete CDC frames
+- they also contain per-packet payload helpers
+
+### Node 2: unified protocol entry
+
+Files added in this step:
+
+- `KeyBorad/KeyBorad/COM/Com_ProtoCodec.h`
+- `KeyBorad/KeyBorad/COM/Com_ProtoCodec.cpp`
+
+Design notes:
+
+- provides a thin and readable entry point for higher layers
+- keeps `DRI` from depending on packet parsing
+- keeps `LGC` from depending on low-level byte layout details
+- starts from the smallest useful handshake path
+
+Implemented behavior:
+
+- build `HelloReq` frame
+- validate whether a parsed packet is `HelloRsp`
+- decode `HelloRsp`
+- forward frame parsing and stream extraction to the existing low-level codec
+
+## Next COM nodes
+
+- bitmap encode/decode helper
+- ack/error helper
+- time sync and theme helper
+- replace direct `Com_Cdc*` calls in higher layers with `Com_ProtoCodec*`