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/04-private-src-not-locked.mut

@@ -0,0 +1,8 @@
+# what: the chmod -R go-rwx on the cloned private source, removed
+# why:  the clone lands at the caller umask -- 022 on stock Ubuntu -- holding ssh config and machine identity
+# kills: the private source is unreadable to anyone else
+# run:  unit
+<<<OLD
+			[ ! -d "$PRIV_SRC" ] || chmod -R go-rwx "$PRIV_SRC" 2>/dev/null || :
+<<<NEW
+<<<END