ocaml: phase 4 'arr.(i)' and 'arr.(i) <- v' array indexing (+3 tests, 515 total)

parse-atom-postfix's '.()' branch now disambiguates between let-open
and array-get based on whether the head is a module path (':con' or
':field' chain rooted in ':con'). Module paths still emit
(:let-open M EXPR); everything else emits (:array-get ARR I).

Eval handles :array-get by reading the cell's underlying list at
index. The '<-' assignment handler now also accepts :array-get lhs
and rewrites the cell with one position changed.

Idiomatic OCaml array code now works:

  let a = Array.make 5 0 in
  for i = 0 to 4 do a.(i) <- i * i done;
  a.(3) + a.(4)               = 25

  let a = Array.init 4 (fun i -> i + 1) in
  a.(0) + a.(1) + a.(2) + a.(3)  = 10

  List.(length [1;2;3])         = 3   (* unchanged: List is a module *)

lib/ocaml/test.sh

@@ -1278,6 +1278,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 5022)
 (eval "(ocaml-run \"List.map ((-) 10) [1;2;3]\")")
 
+;; ── arr.(i) and arr.(i) <- v ────────────────────────────────
+(epoch 5030)
+(eval "(ocaml-run \"let a = Array.make 5 7 in a.(2) <- 99; a.(2) + a.(0)\")")
+(epoch 5031)
+(eval "(ocaml-run \"let a = Array.init 4 (fun i -> i + 1) in a.(0) + a.(1) + a.(2) + a.(3)\")")
+(epoch 5032)
+(eval "(ocaml-run \"let a = Array.make 5 0 in for i = 0 to 4 do a.(i) <- i * i done; a.(3) + a.(4)\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -2030,6 +2038,11 @@ check 5020 "fold_left (+) sum"            '15'
 check 5021 "let f = (*) in f 6 7"         '42'
 check 5022 "List.map ((-) 10) [1;2;3]"    '(9 8 7)'
 
+# ── arr.(i) and arr.(i) <- v ────────────────────────────────────
+check 5030 "a.(2) <- 99; a.(2) + a.(0)"   '106'
+check 5031 "Array.init 4 + sum a.(0..3)"  '10'
+check 5032 "for + a.(i) <- i*i + sum"     '25'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"