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/16-fzf-bad-base-url.mut

@@ -0,0 +1,9 @@
+# what: install_fzf's base URL pointed at a host that does not exist
+# why:  the picker cannot draw the list that installs fzf without fzf
+# kills: the fetch goes to the fzf release download URL
+# run:  unit
+<<<OLD
+	base=https://github.com/junegunn/fzf/releases/download
+<<<NEW
+	base=https://github.invalid/junegunn/fzf/releases/download
+<<<END