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/12-find-tool-skips-local-bin.mut

@@ -0,0 +1,9 @@
+# what: find_tool no longer looks in ~/.local/bin
+# why:  uv, bws and chezmoi all land there, on no PATH the running process has
+# kills: uv is found in ~/.local/bin, off PATH
+# run:  unit
+<<<OLD
+	for ft_c in "$HOME/.local/bin/$1" "$HOME/bin/$1" "$HOME/.npm-global/bin/$1" \
+<<<NEW
+	for ft_c in "$HOME/bin/$1" "$HOME/.npm-global/bin/$1" \
+<<<END