ocaml: phase 2 references ref/!/:= (+6 tests, 189 total)

ref is a builtin boxing its arg in a one-element list. Prefix ! parses
to (:deref ...) and reads via (nth cell 0). := joins the binop
precedence table at level 1 right-assoc and mutates via set-nth!.
Closures share the underlying cell.

lib/ocaml/test.sh

@@ -481,6 +481,20 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 560)
 (eval "(ocaml-run \"match Pair (1, 2) with | Pair (a, b) -> a * b\")")
 
+;; ── References (ref / ! / :=) ──────────────────────────────────
+(epoch 600)
+(eval "(ocaml-run \"let r = ref 5 in !r\")")
+(epoch 601)
+(eval "(ocaml-run \"let r = ref 5 in r := 10; !r\")")
+(epoch 602)
+(eval "(ocaml-run \"let r = ref 0 in r := !r + 1; r := !r + 1; !r\")")
+(epoch 603)
+(eval "(ocaml-run \"let r = ref 100 in let f x = r := !r + x in f 5; f 10; !r\")")
+(epoch 604)
+(eval "(ocaml-run \"let r = ref \\\"a\\\" in r := \\\"b\\\"; !r\")")
+(epoch 605)
+(eval "(ocaml-run \"let count = ref 0 in let rec loop n = if n = 0 then !count else (count := !count + n; loop (n - 1)) in loop 5\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 60 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -767,6 +781,14 @@ check 551 "match x -> x+1"               '100'
 # ctor with tuple arg
 check 560 "Pair(a,b) → a*b"              '2'
 
+# ── References ──────────────────────────────────────────────────
+check 600 "deref new ref"                '5'
+check 601 ":= then deref"                '10'
+check 602 "increment cell twice"         '2'
+check 603 "ref captured by closure"      '115'
+check 604 "ref of string"                '"b"'
+check 605 "ref + recursion"              '15'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"