Mutating the swap step's guard (if [ "$DO_BUILD" = 1 ] -> if false) left the
whole test suite green: test_swap_ordered_after_wait_and_before_start only
checks source positions, which an in-place if-condition edit never moves.
Extracts the decision into should_swap(do_build), following the same shape as
#510's wait_for_daemon_exit and #517's drain_gate_refusal, with a direct test
for each value.
Dai Ha
c89a375e5d