FirmwareEngine: RPi/QEMU backend moved into C++ (no Python host)

[Kinin-Code-Offical]

Goal: Final architecture has ONE FirmwareEngine executable hosting BOTH Arduino (AVR) and Raspberry Pi (QEMU) execution paths.

Scope:

• Move the current RPi shared-memory host loop out of tools/rpi/*.py and into FirmwareEngine (C++), behind a backend interface.
• Remove/disable any 'mock frame pattern' behavior in the production path.
• If RPi board is enabled but QEMU is missing/unavailable, mark that board unavailable (no fake data).

Acceptance:

• FirmwareEngine can start/stop/restart QEMU and manage the shared-memory channels.
• Unity/CoreSim side only talks to FirmwareEngine via the existing IPC contract; no direct Python host process.
• Documentation updated (file hierarchy + responsibilities).