Constant READY_PORT 

pub const READY_PORT: u32 = 51 /* 51u32 */;

Host-side port the guest agent dials once it is fully serving.

Firecracker hybrid vsock forwards a guest-initiated connect to host port P onto the host Unix socket at {uds_path}_{P}, so a pre-bound listener’s accept() IS the “vm-agent is up” event — no connect polling involved.