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/runtime.sx

@@ -661,3 +661,30 @@
       (make-array
         (cons (+ (first a-s) (first b-s)) (rest a-s))
         (append a-r b-r)))))
+
+(define
+  apl-squad
+  (fn
+    (idx-arr data-arr)
+    (let
+      ((shape (get data-arr :shape))
+        (ravel (get data-arr :ravel))
+        (strides (apl-strides (get data-arr :shape))))
+      (let
+        ((idxs (if (scalar? idx-arr) (list (disclose idx-arr)) (get idx-arr :ravel))))
+        (let
+          ((k (len idxs)) (rank (len shape)))
+          (let
+            ((adj (map (fn (i) (- i apl-io)) idxs)))
+            (if
+              (= k rank)
+              (apl-scalar (nth ravel (apl-multi->flat adj strides)))
+              (let
+                ((remaining-shape (drop shape k))
+                  (start (apl-multi->flat adj (take strides k)))
+                  (slice-size (reduce * 1 (drop shape k))))
+                (make-array
+                  remaining-shape
+                  (map
+                    (fn (j) (nth ravel (+ start j)))
+                    (range 0 slice-size)))))))))))