Split CAN comms into source and header files

[yuzeguitarist]

Summary

This is a small, mechanical source/header split toward #2171:

• move the CAN host transport implementation out of board/can_comms.h and into board/can_comms.c
• keep board/can_comms.h declaration-only
• move main firmware global state definitions from board/main_definitions.h into board/main_state.c
• extract the CAN comms bulk-transfer sizing constants into board/comms_constants.h
• wire the new C files into the firmware and tests/libpanda builds

No protocol or behavior changes are intended.

Validation

Local:

• scons -j2
• ruff check .
• pytest tests/usbprotocol/test_comms.py tests/usbprotocol/test_pandalib.py -q
• pytest -q --ignore=tests/misra
• git diff --check

GitHub Actions:

• ./test.sh (macos-latest) passed
• ./test.sh (ubuntu-latest) passed
• build passed
• windows pip package test passed

Refs #2171