Two sessions checked each other and each found what the other missed. I
measured that the tree was not waiting on a commit; the peer re-derived that
independently, then added the two checks my recommendation lacked -- that HEAD
is a true ancestor (so it is a real fast-forward, not a merge in disguise) and
that the branch touches none of PLN's five modified .tidal files.

That second one is the one that mattered. Recommending a checkout across
somebody's live set prep without measuring the direction is how a repair
becomes a clobber, and I had asserted the switch was clean on the strength of
the two tools files alone.

Both verified here before recording them. The caution that remains is real and
unchanged: the fast-forward swaps code under a running rig.