Add firmware nanopb protocol core

docs/firmware_proto_transport.md

@@ -32,11 +32,6 @@ Design notes:
 
 ## Planned next nodes
 
-- keyboard split logic uses `function_bitmap_event`
-- CDC transport module
-- GATT transport module
-- nanopb integration and generated protocol code
-
 ### Node 2: keyboard split point
 
 Files updated in this step:
@@ -59,3 +54,43 @@ Implemented behavior:
 - for consumer usages marked as function keys:
   - stop normal HID reporting
   - emit `function_key_event`
+
+### Node 3: nanopb build integration and protocol core
+
+Files updated in this step:
+
+- `CMakeLists.txt`
+- `prj.conf`
+- `app.overlay`
+- `inc/keyboard_proto.h`
+- `src/modules/keyboard_proto.c`
+
+Design notes:
+
+- use NCS built-in nanopb instead of a hand-written protobuf runtime
+- generate protocol code from the shared `.proto`
+- keep one firmware-side protocol helper that:
+  - encodes and decodes `CdcPacketBody`
+  - encodes and decodes `CdcFrame`
+  - validates checksum
+  - routes host commands back into existing modules
+
+Implemented behavior:
+
+- enable CDC ACM class in Zephyr USB device-next stack
+- add a CDC ACM UART node in devicetree
+- wire `zephyr_nanopb_sources()` into the build
+- add protocol helper functions for:
+  - body encode/decode
+  - CDC frame encode
+  - CDC frame stream extraction
+  - hello response build
+  - ack/error build
+  - function key event build
+  - LED state build
+  - host command dispatch
+
+## Planned next nodes
+
+- CDC transport module
+- GATT transport module