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;

test/spec/sequences-spec.janet

@@ -145,3 +145,23 @@
   ["symbol from keyword"  "\"x\"" "(name (symbol :x))"]
   ["keyword bad 2-arg"    :throws "(keyword \"abc\" nil)"]
   ["keyword from symbol"  "\"x\"" "(name (keyword 'x))"])
+
+# Stack/accessor strictness: peek/pop are stack-only; vec needs a seqable;
+# key/val need a map entry.
+(defspec "seq / accessor strictness"
+  ["peek vector"        "3"      "(peek [1 2 3])"]
+  ["peek list"          "1"      "(peek '(1 2 3))"]
+  ["peek empty vec"     "nil"    "(peek [])"]
+  ["peek on set"        :throws  "(peek #{1 2})"]
+  ["peek on number"     :throws  "(peek 42)"]
+  ["pop empty vec"      :throws  "(pop [])"]
+  ["pop on number"      :throws  "(pop 0)"]
+  ["pop vector"         "[1 2]"  "(pop [1 2 3])"]
+  ["vec on number"      :throws  "(vec 42)"]
+  ["vec on keyword"     :throws  "(vec :a)"]
+  ["vec ok"             "[1 2]"  "(vec '(1 2))"]
+  ["key on nil"         :throws  "(key nil)"]
+  ["key on map"         :throws  "(key {})"]
+  ["val on number"      :throws  "(val 0)"]
+  ["key of entry"       ":a"     "(key (first {:a 1}))"]
+  ["val of entry"       "1"      "(val (first {:a 1}))"])