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/tests/structural.sx

@@ -434,4 +434,44 @@
 (apl-test
   "squad partial 3d slice shape"
   (sh (apl-squad (apl-scalar 1) (make-array (list 2 3 4) (range 1 25))))
-  (list 3 4))
+  (list 3 4))
+
+(apl-test
+  "grade-up basic"
+  (rv (apl-grade-up (make-array (list 5) (list 3 1 4 1 5))))
+  (list 2 4 1 3 5))
+
+(apl-test
+  "grade-up shape"
+  (sh (apl-grade-up (make-array (list 4) (list 4 1 3 2))))
+  (list 4))
+
+(apl-test
+  "grade-up no duplicates"
+  (rv (apl-grade-up (make-array (list 4) (list 4 1 3 2))))
+  (list 2 4 3 1))
+
+(apl-test
+  "grade-up already sorted"
+  (rv (apl-grade-up (make-array (list 3) (list 1 2 3))))
+  (list 1 2 3))
+
+(apl-test
+  "grade-up reverse sorted"
+  (rv (apl-grade-up (make-array (list 3) (list 3 2 1))))
+  (list 3 2 1))
+
+(apl-test
+  "grade-down basic"
+  (rv (apl-grade-down (make-array (list 5) (list 3 1 4 1 5))))
+  (list 5 3 1 2 4))
+
+(apl-test
+  "grade-down no duplicates"
+  (rv (apl-grade-down (make-array (list 4) (list 4 1 3 2))))
+  (list 1 3 4 2))
+
+(apl-test
+  "grade-up single element"
+  (rv (apl-grade-up (make-array (list 1) (list 42))))
+  (list 1))