apl: Phase 3 squad ⌷ indexing — 66/66 tests green

Add apl-squad: scalar index into vector, fully-specified multi-dim index,
partial index returning sub-array slice. 7 new tests.

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

lib/apl/tests/structural.sx

@@ -388,4 +388,50 @@
     (apl-catenate-first
       (make-array (list 2 3) (list 1 2 3 4 5 6))
       (make-array (list 3 3) (list 11 12 13 14 15 16 17 18 19))))
-  (list 1 2 3 4 5 6 11 12 13 14 15 16 17 18 19))
+  (list 1 2 3 4 5 6 11 12 13 14 15 16 17 18 19))
+
+(apl-test
+  "squad scalar into vector"
+  (rv
+    (apl-squad (apl-scalar 2) (make-array (list 5) (list 10 20 30 40 50))))
+  (list 20))
+
+(apl-test
+  "squad first element"
+  (rv (apl-squad (apl-scalar 1) (make-array (list 3) (list 10 20 30))))
+  (list 10))
+
+(apl-test
+  "squad last element"
+  (rv
+    (apl-squad (apl-scalar 5) (make-array (list 5) (list 10 20 30 40 50))))
+  (list 50))
+
+(apl-test
+  "squad fully specified matrix element"
+  (rv
+    (apl-squad
+      (make-array (list 2) (list 2 3))
+      (make-array (list 3 4) (list 1 2 3 4 5 6 7 8 9 10 11 12))))
+  (list 7))
+
+(apl-test
+  "squad partial row of matrix shape"
+  (sh
+    (apl-squad
+      (apl-scalar 2)
+      (make-array (list 3 4) (list 1 2 3 4 5 6 7 8 9 10 11 12))))
+  (list 4))
+
+(apl-test
+  "squad partial row of matrix ravel"
+  (rv
+    (apl-squad
+      (apl-scalar 2)
+      (make-array (list 3 4) (list 1 2 3 4 5 6 7 8 9 10 11 12))))
+  (list 5 6 7 8))
+
+(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))