apl: multi-axis bracket A[I;J] / A[I;] / A[;J] (+8 tests, 475/475)

Parser: split-bracket-content splits inner tokens on :semi at
depth 0; maybe-bracket emits (:bracket arr axis-exprs...) for
multi-axis access, with :all marker for empty axes.

Runtime: apl-bracket-multi enumerates index combinations via
apl-cartesian (helper) and produces sub-array. Scalar axes
collapse from result shape; vector / nil axes contribute their
length.

apl-run "M ← (3 3) ⍴ ⍳9 ⋄ M[2;2]"  → 5
apl-run "M ← (3 3) ⍴ ⍳9 ⋄ M[1;]"   → 1 2 3
apl-run "M ← (3 3) ⍴ ⍳9 ⋄ M[;2]"   → 2 5 8
apl-run "M ← (2 3) ⍴ ⍳6 ⋄ M[1 2;1 2]" → 2x2 sub-block

lib/apl/runtime.sx

@@ -985,6 +985,38 @@
         (some (fn (c) (= c 0)) codes)
         (some (fn (c) (= c (nth e 1))) codes)))))
 
+(define
+  apl-cartesian
+  (fn
+    (lists)
+    (if
+      (= (len lists) 0)
+      (list (list))
+      (let
+        ((rest-prods (apl-cartesian (rest lists))))
+        (reduce
+          (fn (acc x) (append acc (map (fn (p) (cons x p)) rest-prods)))
+          (list)
+          (first lists))))))
+
+(define
+  apl-bracket-multi
+  (fn
+    (axes arr)
+    (let
+      ((shape (get arr :shape)) (ravel (get arr :ravel)))
+      (let
+        ((rank (len shape)) (strides (apl-strides shape)))
+        (let
+          ((axis-info (map (fn (i) (let ((a (nth axes i))) (cond ((= a nil) {:idxs (range 0 (nth shape i)) :scalar? false}) ((= (len (get a :shape)) 0) {:idxs (list (- (first (get a :ravel)) apl-io)) :scalar? true}) (else {:idxs (map (fn (x) (- x apl-io)) (get a :ravel)) :scalar? false})))) (range 0 rank))))
+          (let
+            ((cells (apl-cartesian (map (fn (a) (get a :idxs)) axis-info))))
+            (let
+              ((result-ravel (map (fn (cell) (let ((flat (reduce + 0 (map (fn (i) (* (nth cell i) (nth strides i))) (range 0 rank))))) (nth ravel flat))) cells)))
+              (let
+                ((result-shape (filter (fn (x) (>= x 0)) (map (fn (i) (let ((a (nth axis-info i))) (if (get a :scalar?) -1 (len (get a :idxs))))) (range 0 rank)))))
+                (make-array result-shape result-ravel)))))))))
+
 (define
   apl-reduce
   (fn