Add firmware GATT protocol module

docs/firmware_proto_transport.md

@@ -119,3 +119,29 @@ Implemented behavior:
   - `Error`
   - `FunctionKeyEvent`
   - `LedState`
+
+### Node 5: GATT transport module
+
+Files added in this step:
+
+- `src/modules/ble_gatt_proto_module.c`
+
+Design notes:
+
+- define one custom service with one RX write characteristic and one TX notify
+  characteristic
+- keep GATT transport byte-oriented by sending raw `CdcPacketBody` bytes
+- reuse the same host command dispatcher as CDC
+
+Implemented behavior:
+
+- register the custom GATT service
+- accept host writes on RX
+- decode `CdcPacketBody`
+- dispatch host messages
+- notify:
+  - `HelloRsp`
+  - `Ack`
+  - `Error`
+  - `FunctionKeyEvent`
+  - `LedState`