Refactor panel protocol parsing

[RhythrosaLabs]

Summary

Refactors the five-byte panel MCU parser to use typed packed packet views and a shared big-endian 32-bit decoder.

The change:

• names the button, encoder, knob, trigger, XY, acknowledgement, and held-button packet fields
• removes repeated raw-byte indexing and duplicated word assembly
• preserves callback behavior and wire-format handling
• keeps byte-order conversion explicit and portable

Resolves #43.

Validation

• cc -std=c11 -Wall -Wextra -Werror -fsyntax-only passes with the repository firmware include paths.
• git diff --check passes.
• make -C cpu APP=demo is currently blocked because arm-none-eabi-gcc is not installed in the development environment.

The implementation is also published on RhythrosaLabs/freetribe-studio main as commit 78e4c4f.