Wraps a value so it broadcasts against the trailing axes in listable operations
與 .wl 參考的差異： `Threaded[scalar]` gives garbage
Threaded[list]
a +Threaded[b]
a +Threaded[b, alev]
a +Threaded[b, blev -> alev]
f[a, Threaded[b, …]]
{{1, 2}, {3, 4}, {5, 6}} + Threaded[{x, y}]
→ {{1 + x, 2 + y}, {3 + x, 4 + y}, {5 + x, 6 + y}}2 * (Threaded[{-1, 1}] * {{a, b}, {c, d}})
→ {{-2*a, 2*b}, {-2*c, 2*d}}