ocaml: phase 6 Array module + (op) operator sections (+6 tests, 512 total)

Array module (runtime.sx, OCaml syntax):
  Backed by a 'ref of list'. make/length/get/init build the cell;
  set rewrites the underlying list with one cell changed (O(n) but
  works for short arrays in baseline programs). Includes
  iter/iteri/map/mapi/fold_left/to_list/of_list/copy/blit/fill.

(op) operator sections (parser.sx, parse-atom):
  When the token after '(' is a binop (any op with non-zero
  precedence in the binop table) and the next token is ')', emit
  (:fun ('a' 'b') (:op OP a b)) — i.e. (+)  becomes fun a b -> a + b.
  Recognises every binop including 'mod', 'land', '^', '@', '::',
  etc.

Lets us write:
  List.fold_left (+) 0 [1;2;3;4;5]    = 15
  let f = ( * ) in f 6 7              = 42
  List.map ((-) 10) [1;2;3]           = [9;8;7]
  let a = Array.make 5 7 in
  Array.set a 2 99;
  Array.fold_left (+) 0 a             = 127

plans/ocaml-on-sx.md

@@ -407,6 +407,16 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 6 — Array module (ref-of-list backing) + (op)
+  operator sections (+6 tests, 512 total). Array implements
+  make/length/get/set/init/iter/iteri/map/mapi/fold_left/to_list/
+  of_list/copy/blit/fill in OCaml syntax in runtime.sx; backing is a
+  `ref of list` so set is O(n) but mutation works. (op) sections in
+  parse-atom: when the token after `(` is a binop and the next is
+  `)`, emit `(:fun ("a" "b") (:op OP a b))` — `(+)` becomes `fun a b
+  -> a + b`. Recognises any binop in the precedence table including
+  `mod`, `land`, `^`, `@`, `::`, etc. Lets us write `List.fold_left
+  (+) 0 xs` and `((-) 10)` partial applications.
 - 2026-05-09 Phase 5.1 — csv.ml baseline (split on '\n' then ',',
   parse-int the second field, fold-left). Exercises char escapes
   inside string literals, two-stage String.split_on_char, mixed