Add firmware CDC protocol module

docs/firmware_proto_transport.md

@@ -92,5 +92,30 @@ Implemented behavior:
 
 ## Planned next nodes
 
-- CDC transport module
 - GATT transport module
+
+### Node 4: CDC transport module
+
+Files added in this step:
+
+- `src/modules/usb_cdc_proto_module.c`
+
+Design notes:
+
+- use the CDC ACM UART abstraction from Zephyr
+- keep CDC transport separate from protocol details by delegating encode/decode
+  to `keyboard_proto`
+- report private outgoing events back to host over CDC
+
+Implemented behavior:
+
+- receive raw CDC bytes by UART IRQ
+- accumulate bytes into a stream buffer
+- extract protobuf `CdcFrame`
+- decode host messages and dispatch them
+- send:
+  - `HelloRsp`
+  - `Ack`
+  - `Error`
+  - `FunctionKeyEvent`
+  - `LedState`