feat(strictness): peek/pop/vec/key/val reject malformed args

- 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.

test/integration/clojure-test-suite-test.janet

@@ -18,7 +18,7 @@
 
 # Baseline: assertions Jolt currently passes across the suite. Raise as Jolt
 # improves so a regression (previously-passing assertion breaking) is caught.
-(def baseline-pass 3820)
+(def baseline-pass 3835)
 # A file is "clean" when it ran with zero failures AND zero errors.
 (def baseline-clean-files 45)
 # Per-file wall-clock budget (seconds). Normal files finish in well under 1s;