reservedGensAsc() documented but never enforced that pending single-op
generations must sort above every committed one. A direct
commitTransaction() call bypassing Brainy.transact()'s flush-first step
could commit a fresh generation into committedRanges above lower,
still-pending ones, unsorting the committed-then-pending walk
resolveManyAt relies on and returning a wrong before-image for a
point-in-time read — silently. commitTransaction() now refuses via a new
PendingSingleOpsUnflushedError when the pending tier is non-empty, before
any staging I/O. Behavior-neutral: both sanctioned callers
(Brainy.transact(), Brainy.compactHistory()) already flush first.
