Constant CAP_HANDLE_PASSING 

pub const CAP_HANDLE_PASSING: u64 = _; // 1u64

Capability bit: the peer can participate in broker-to-backend connection handoff via platform handle passing (DuplicateHandle on Windows, SCM_RIGHTS on Unix).

When negotiated, the broker issues a one-time pending handoff token in Negotiated.handle_passed_token. Actual handle adoption is a later slice; reconnecting to Negotiated.backend_pipe remains the correctness path.