Receiver: return bool from receive() to indicate onReceive handler result

[Tyagiquamar]

Fixes #92.

Summary of Changes

• Updated Receiver::receive() return type from void to bool across protocol headers and implementation (proto/fbe_protocol.h, proto/fbe_protocol.cpp).
• Updated C++ code generator template in source/generator_cpp.cpp to generate bool-returning receive() methods.
• Forwarded onReceive() handler boolean return value through receive(), allowing callers to detect message handling failures and malformed payloads.