apl: Phase 3 catenate , and first-axis — 59/59 tests green

Add apl-catenate (dyadic ,, last-axis join, scalar promotion) and
apl-catenate-first (first-axis join, row-major append). 9 new tests.

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

lib/apl/tests/structural.sx

@@ -322,4 +322,70 @@
     (apl-rotate-first
       (apl-scalar 1)
       (make-array (list 2 3) (list 1 2 3 4 5 6))))
-  (list 4 5 6 1 2 3))
+  (list 4 5 6 1 2 3))
+
+(apl-test
+  "cat v,v ravel"
+  (rv
+    (apl-catenate
+      (make-array (list 3) (list 1 2 3))
+      (make-array (list 2) (list 4 5))))
+  (list 1 2 3 4 5))
+
+(apl-test
+  "cat v,v shape"
+  (sh
+    (apl-catenate
+      (make-array (list 3) (list 1 2 3))
+      (make-array (list 2) (list 4 5))))
+  (list 5))
+
+(apl-test
+  "cat scalar,v"
+  (rv (apl-catenate (apl-scalar 99) (make-array (list 3) (list 1 2 3))))
+  (list 99 1 2 3))
+
+(apl-test
+  "cat v,scalar"
+  (rv (apl-catenate (make-array (list 3) (list 1 2 3)) (apl-scalar 99)))
+  (list 1 2 3 99))
+
+(apl-test
+  "cat matrix last-axis shape"
+  (sh
+    (apl-catenate
+      (make-array (list 2 3) (list 1 2 3 4 5 6))
+      (make-array (list 2 2) (list 7 8 9 10))))
+  (list 2 5))
+
+(apl-test
+  "cat matrix last-axis ravel"
+  (rv
+    (apl-catenate
+      (make-array (list 2 3) (list 1 2 3 4 5 6))
+      (make-array (list 2 2) (list 7 8 9 10))))
+  (list 1 2 3 7 8 4 5 6 9 10))
+
+(apl-test
+  "cat-first v,v shape"
+  (sh
+    (apl-catenate-first
+      (make-array (list 3) (list 1 2 3))
+      (make-array (list 2) (list 4 5))))
+  (list 5))
+
+(apl-test
+  "cat-first matrix shape"
+  (sh
+    (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 5 3))
+
+(apl-test
+  "cat-first matrix ravel"
+  (rv
+    (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))