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/18-deb-never-installed.mut

@@ -0,0 +1,8 @@
+# what: install_deb's install step, removed: it downloads and stops
+# why:  chrome and ghostty would report success having installed nothing
+# kills: …and then handed to the package manager
+# run:  unit
+<<<OLD
+		run_sh "${SUDO:+$SUDO }apt-get install -y '$f'" || note_fail "$(keyof deb "$s")" "dpkg install failed"
+<<<NEW
+<<<END