ocaml: phase 1 type annotations on let / (e : T) (+4 tests, 473 total)

let NAME [PARAMS] : T = expr and (expr : T) parse and skip the type
source. Runtime no-op since SX is dynamic. Works in inline let,
top-level let, and parenthesised expressions:

  let x : int = 5 ;; x + 1                        -> 6
  let f (x : int) : int = x + 1 in f 41           -> 42
  (5 : int)                                       -> 5
  ((1 + 2) : int) * 3                             -> 9

lib/ocaml/test.sh

@@ -1162,6 +1162,16 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4601)
 (eval "(ocaml-run-program \"type t = int;; 42\")")
 
+;; ── Type annotations: let x : T = e and (e : T) ──────────────
+(epoch 4700)
+(eval "(ocaml-run-program \"let x : int = 5;; x + 1\")")
+(epoch 4701)
+(eval "(ocaml-run \"let f (x : int) : int = x + 1 in f 41\")")
+(epoch 4702)
+(eval "(ocaml-run \"(5 : int)\")")
+(epoch 4703)
+(eval "(ocaml-run \"((1 + 2) : int) * 3\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1841,6 +1851,12 @@ check 4502 "try when fall through"        '1005'
 check 4600 "type t = int parses"          '("type-alias" "t" ())'
 check 4601 "type alias decl + use"        '42'
 
+# ── Type annotations ───────────────────────────────────────────
+check 4700 "let x : int = 5"              '6'
+check 4701 "let f (x : int) : int"        '42'
+check 4702 "(5 : int)"                    '5'
+check 4703 "((1+2) : int) * 3"            '9'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"