apl: Phase 3 grade-up ⍋ / grade-down ⍒ — 74/74 tests green

Add apl-grade (stable insertion sort helper), apl-grade-up, apl-grade-down.
Stability guaranteed via secondary sort key (original index). 8 new tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/apl/runtime.sx

@@ -688,3 +688,47 @@
                   (map
                     (fn (j) (nth ravel (+ start j)))
                     (range 0 slice-size)))))))))))
+
+(define
+  apl-grade
+  (fn
+    (arr ascending)
+    (let
+      ((ravel (get arr :ravel)) (n (len (get arr :ravel))))
+      (let
+        ((pairs (map (fn (i) (list (nth ravel i) (+ i apl-io))) (range 0 n))))
+        (define ins nil)
+        (set!
+          ins
+          (fn
+            (x sorted)
+            (if
+              (= (len sorted) 0)
+              (list x)
+              (let
+                ((xv (first x))
+                  (xi (nth x 1))
+                  (hd (first sorted))
+                  (sv (first hd))
+                  (si (nth hd 1)))
+                (if
+                  (if
+                    ascending
+                    (or (< xv sv) (and (= xv sv) (< xi si)))
+                    (or (> xv sv) (and (= xv sv) (< xi si))))
+                  (cons x sorted)
+                  (cons hd (ins x (rest sorted))))))))
+        (define isort nil)
+        (set!
+          isort
+          (fn
+            (lst)
+            (if
+              (= (len lst) 0)
+              (list)
+              (ins (first lst) (isort (rest lst))))))
+        (make-array (list n) (map (fn (p) (nth p 1)) (isort pairs)))))))
+
+(define apl-grade-up (fn (arr) (apl-grade arr true)))
+
+(define apl-grade-down (fn (arr) (apl-grade arr false)))