Each file in .tests/mutations/ is one deliberate bug: what it breaks, why that
matters, the assertion meant to catch it, and an OLD/NEW pair applied to a copy
of the tree. A mutation is KILLED only when the suite fails AND the named
assertion is among the failures — failing for an unrelated reason is reported
as WRONG-TEST, because that is luck, and luck is lost the next time the
unrelated test moves. A mutation whose OLD block no longer matches is STALE
rather than quietly skipped.
19 reproduces the DU-C1 shape: a comment moved back inside the fzf
line-continuation. It is killed by "dotup pick exits 0".
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