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/19-pick-comment-in-continuation.mut

@@ -0,0 +1,10 @@
+# what: a comment moved back INSIDE the fzf line-continuation (the DU-C1 shape)
+# why:  the `\`-newline is stripped first, so the comment's own newline ends the command: fzf gets two binds and the lines below run as a command named --bind
+# kills: dotup pick exits 0
+# run:  unit
+<<<OLD
+		--bind "tab:execute-silent($SELF expand {2})+reload($SELF render)" \
+<<<NEW
+		--bind "tab:execute-silent($SELF expand {2})+reload($SELF render)" \
+		# clear-query is not cosmetic: ^t toggles every row the filter shows.
+<<<END