Add process-targeted messaging and scoped RPC caller identity

[imakris]

Summary

• add typed emission targeted to one exact process
• fix the first transported RPC deadlock by resolving message type before acquiring outer RPC state
• expose scoped caller process identity for authorization inside transported RPC handlers

Verification

• deterministic pre-fix first-RPC deadlock oracle now passes
• caller identity is exact per process, isolated across nested/concurrent handlers, and cleared on return and exception
• Windows MSVC and Ubuntu 24.04 GCC gates passed

[imakris]

Superseded by the clean, master-bound replacement in #857. This PR must not be merged.