Add firmware function events

docs/firmware_proto_transport.md

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+# Firmware Proto Transport
+
+## Goal
+
+Add CDC and GATT private communication without rewriting the existing keyboard,
+time, theme, and LED business logic.
+
+The firmware side should:
+
+- keep standard HID behavior for normal keys
+- receive host commands over CDC and GATT
+- report private state and function key events over CDC and GATT
+- reuse existing modules where possible
+
+## Completed Nodes
+
+### Node 1: internal function events
+
+Files:
+
+- `src/events/function_bitmap_event.h`
+- `src/events/function_bitmap_event.c`
+- `src/events/function_key_event.h`
+- `src/events/function_key_event.c`
+
+Design notes:
+
+- `function_bitmap_event` carries the 29-byte function bitmap from host
+- `function_key_event` carries usage + action when a configured function key
+  is pressed or released
+- these events isolate transport modules from keyboard internals
+
+## Planned next nodes
+
+- keyboard split logic uses `function_bitmap_event`
+- CDC transport module
+- GATT transport module
+- nanopb integration and generated protocol code