- peek/pop are stack-only (vectors/lists): throw on sets/maps/strings/scalars,
and pop throws on an empty vector/list
- vec throws on non-seqable args (numbers/keywords/transients)
- key/val require a map entry (2-element vector); throw on nil/numbers/maps/sets
pop clean; peek 9/2; vec 17/2/1; key/val 12/2/1 (remaining are the
2-vector-vs-MapEntry / tuple-from-seq divergences). pass 3824->3840.
spec: seq/accessor-strictness (16). jpm test green.
b82477dac3