RPi SHM protocol: make C/C++ header the single source of truth

[Kinin-Code-Offical]

Goal: The shared-memory header/payload layout for RaspberryPi channels must be defined once, in C/C++ (source of truth), and used by all producers/consumers.

Scope:

• Add a protocol header (e.g. RpiShmProtocol.h) in FirmwareEngine that defines MAGIC/version/header struct.
• Provide packing/unpacking helpers.
• Optional: codegen or mirrored definition for C# (Unity) to avoid drift.

Acceptance:

• No duplicated 'magic/version/header size' definitions across Python/C#/C++ in production path.
• Versioning strategy for backward compatibility.

[github-actions]

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