kernel: variadic +-*/, chained <>=? + 19 tests [nothing]

knl-fold-app for n-ary fold with zero-arity identity and one-arity
special-case (- negates, / inverts). knl-chain-cmp for chained
boolean comparison. 279 tests total.

lib/kernel/tests/standard.sx

@@ -337,4 +337,30 @@
 (ks-test "or: middle truthy"
   (ks-eval "($or? #f 42 nope)") 42)
 
+;; ── variadic arithmetic ─────────────────────────────────────────
+(ks-test "+: zero args = 0"    (ks-eval "(+)") 0)
+(ks-test "+: one arg = arg"    (ks-eval "(+ 7)") 7)
+(ks-test "+: two args"         (ks-eval "(+ 3 4)") 7)
+(ks-test "+: five args"        (ks-eval "(+ 1 2 3 4 5)") 15)
+
+(ks-test "*: zero args = 1"    (ks-eval "(*)") 1)
+(ks-test "*: one arg"          (ks-eval "(* 7)") 7)
+(ks-test "*: four args"        (ks-eval "(* 1 2 3 4)") 24)
+
+(ks-test "-: one arg negates"  (ks-eval "(- 10)") -10)
+(ks-test "-: two args"         (ks-eval "(- 10 3)") 7)
+(ks-test "-: four args fold"   (ks-eval "(- 100 1 2 3)") 94)
+
+(ks-test "/: two args"         (ks-eval "(/ 20 5)") 4)
+(ks-test "/: three args fold"  (ks-eval "(/ 100 2 5)") 10)
+
+;; ── variadic chained comparison ─────────────────────────────────
+(ks-test "<: chained ascending"  (ks-eval "(< 1 2 3 4 5)") true)
+(ks-test "<: not strict"          (ks-eval "(< 1 2 2 3)") false)
+(ks-test "<: anti-monotonic"      (ks-eval "(< 5 3)") false)
+(ks-test ">: chained descending" (ks-eval "(> 5 4 3 2 1)") true)
+(ks-test "<=? ascending equals"   (ks-eval "(<=? 1 1 2 3 3)") true)
+(ks-test "<=? violation"          (ks-eval "(<=? 1 2 1)") false)
+(ks-test ">=? descending equals" (ks-eval "(>=? 3 3 2 1)") true)
+
 (define ks-tests-run! (fn () {:total (+ ks-test-pass ks-test-fail) :passed ks-test-pass :failed ks-test-fail :fails ks-test-fails}))