test: mutation testing — break dotup on purpose and check the suite notices

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".

.tests/mutations/10-note-fail-silent.mut

@@ -0,0 +1,9 @@
+# what: note_fail() neutered: failures are dropped
+# why:  the summary, the count and the exit code all come from this one file
+# kills: …under a heading you can find
+# run:  unit
+<<<OLD
+note_fail() { printf '%s\t%s\n' "$1" "$2" >> "$FAILED"; err "$1: $2"; }
+<<<NEW
+note_fail() { :; }
+<<<END