Add a PREDICATE selector to $<LIST:TRANSFORM> that chooses which elements to
transform by evaluating a <body> per element (with $<_0> bound) and acting on
those for which it yields "1". This selects by computed condition instead of
the fixed AT/FOR/REGEX positions, so any generator expression -- including
target queries -- can decide where an action applies. It works with both the
canned actions and APPLY.
Issue: #27892
