feat(strictness): first/rseq shape checks, assoc even-args + non-associative

- first throws on scalars (numbers/keywords/booleans/char & symbol structs)
- rseq is vector/sorted-only (throws on strings/maps/numbers/seqs)
- assoc requires an even kv count and a map/vector/nil receiver

first clean; rseq 12/1; assoc 39/3. clojure-test-suite pass 3864->3874.
spec: seq/more-strictness (11). 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 3860)
+(def baseline-pass 3870)
 # 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

@@ -165,3 +165,17 @@
   ["val on number"      :throws  "(val 0)"]
   ["key of entry"       ":a"     "(key (first {:a 1}))"]
   ["val of entry"       "1"      "(val (first {:a 1}))"])
+
+# More strictness: first/rseq on the right shapes, assoc even-arg requirement.
+(defspec "seq / more strictness"
+  ["first on number"    :throws "(first 42)"]
+  ["first on keyword"   :throws "(first :a)"]
+  ["first ok vec"       "1"     "(first [1 2])"]
+  ["first ok nil"       "nil"   "(first nil)"]
+  ["rseq vector"        "[3 2 1]" "(rseq [1 2 3])"]
+  ["rseq on string"     :throws "(rseq \"ab\")"]
+  ["rseq on map"        :throws "(rseq {:a 1})"]
+  ["rseq on number"     :throws "(rseq 0)"]
+  ["assoc odd args"     :throws "(assoc {:a 1} :b)"]
+  ["assoc on number"    :throws "(assoc 5 :a 1)"]
+  ["assoc on set"       :throws "(assoc #{} :a 1)"])