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/17-nvim-path-typo.mut

@@ -0,0 +1,9 @@
+# what: the neovim tarball extracted to /opt/nvim-TYPO
+# why:  tar succeeds, the symlink dangles, and nothing says so
+# kills: the tarball is extracted where the symlink points
+# run:  unit
+<<<OLD
+			run_sh "${SUDO:+$SUDO }rm -rf /opt/nvim && ${SUDO:+$SUDO }mkdir -p /opt/nvim && ${SUDO:+$SUDO }tar -xzf /tmp/nvim.tgz -C /opt/nvim --strip-components=1" \
+<<<NEW
+			run_sh "${SUDO:+$SUDO }rm -rf /opt/nvim && ${SUDO:+$SUDO }mkdir -p /opt/nvim && ${SUDO:+$SUDO }tar -xzf /tmp/nvim.tgz -C /opt/nvim-TYPO --strip-components=1" \
+<<<END