Add panel control code example

[RhythrosaLabs]

Summary

Adds a standalone APP=panel-codes example for discovering and documenting panel input codes.

The example:

• registers button, knob, and encoder callbacks
• logs every event with its numeric code and value
• names the currently documented controls
• leaves unknown codes visible as UNKNOWN so hardware discovery can continue safely

Resolves #29.

Validation

• cc -std=c11 -Wall -Wextra -Werror -fsyntax-only passes with the repository firmware include paths.
• make -C cpu APP=panel-codes could not run because arm-none-eabi-gcc is not installed in this environment.

This branch comes from the upstream main baseline and is published from the RhythrosaLabs/freetribe-studio fork.