Internal pipeline packet.
reply_to is the revocable reply destination — a process alias owned
by the caller, unaliased when the call ends, so a late result is dropped
by the VM instead of rotting in a mailbox (FX-005). deadline is the
packet's absolute local monotonic deadline in milliseconds (nil means
it never expires); a stage that would begin after it skips the packet
and marks expired: true, which the caller sees as its own timeout.
Local to one BEAM node — do not treat these fields as a wire protocol.