stepBounds.Arm deleted the key and created it again, so two builder instances waiting on one ask raced: one's
create met the other's and failed "wrong last sequence", and the ask's build failed (the builder bed's two
instances together). Arm now creates the key only if absent; one already there is armed, and nothing deletes
it to make it again.
Builds: design 47 G A-282, A-115, §544
Claude-Session: https://claude.ai/code/session_01NcbgfGKaxw984XyAPfLm4h
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