Fix synthetic SIF RPC server memory aliasing

[GTTeancum]

Summary

• mark placeholder SIF RPC servers as synthetic in host state
• avoid reading mutable guest-memory fields from synthetic descriptors during bind/call
• preserve normal guest server descriptor behavior for registered RPC servers
• add a regression test that overwrites a placeholder descriptor and verifies it cannot redirect a payload copy into unrelated EE memory

Why

Placeholder servers are allocated inside EE RDRAM so guest bind loops receive a nonzero server pointer. A game can later reuse that address. SifCallRpc previously trusted the overwritten descriptor's buf field and copied request data to that stale destination, corrupting unrelated live game objects.

Testing

• MINITEST_FILTER=synthetic RPC server ignores: 1/1 passed
• MINITEST_FILTER=PS2SifRpc: 18/18 passed
• integration boot probe: 578 calls to the affected synthetic controller RPC with zero writes to the previously corrupted object