The agent no longer self-terminates on a 120s deadline: a VMI host may attach
at any time, so the beacon now polls its ack flag indefinitely and exits only
once the host sets it. The fixed lifetime was an artifact, not a requirement.
The contract drops from three constants to one. The companion magic is derived
as the byte-reversed primary (__builtin_bswap64, folded to an immediate at -O2),
giving the same 16-byte beacon signature from a single source of truth; the host
acknowledges by echoing that value into the ack slot instead of carrying a
separate ack constant.