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/tests/pipeline.sx

@@ -242,3 +242,43 @@
   "named-fn with vector arg: neg ← {-⍵} ⋄ neg 1 2 3"
   (mkrv (apl-run "neg ← {-⍵} ⋄ neg 1 2 3"))
   (list -1 -2 -3))
+
+(apl-test
+  "multi-axis: M[2;2] → center"
+  (mkrv (apl-run "M ← (3 3) ⍴ ⍳9 ⋄ M[2;2]"))
+  (list 5))
+
+(apl-test
+  "multi-axis: M[1;] → first row"
+  (mkrv (apl-run "M ← (3 3) ⍴ ⍳9 ⋄ M[1;]"))
+  (list 1 2 3))
+
+(apl-test
+  "multi-axis: M[;2] → second column"
+  (mkrv (apl-run "M ← (3 3) ⍴ ⍳9 ⋄ M[;2]"))
+  (list 2 5 8))
+
+(apl-test
+  "multi-axis: M[1 2;1 2] → 2x2 block"
+  (mkrv (apl-run "M ← (2 3) ⍴ ⍳6 ⋄ M[1 2;1 2]"))
+  (list 1 2 4 5))
+
+(apl-test
+  "multi-axis: M[1 2;1 2] shape (2 2)"
+  (mksh (apl-run "M ← (2 3) ⍴ ⍳6 ⋄ M[1 2;1 2]"))
+  (list 2 2))
+
+(apl-test
+  "multi-axis: M[;] full matrix"
+  (mkrv (apl-run "M ← (2 2) ⍴ 10 20 30 40 ⋄ M[;]"))
+  (list 10 20 30 40))
+
+(apl-test
+  "multi-axis: M[1;] shape collapsed"
+  (mksh (apl-run "M ← (3 3) ⍴ ⍳9 ⋄ M[1;]"))
+  (list 3))
+
+(apl-test
+  "multi-axis: select all rows of column 3"
+  (mkrv (apl-run "M ← (4 3) ⍴ 1 2 3 4 5 6 7 8 9 10 11 12 ⋄ M[;3]"))
+  (list 3 6 9 12))