ocaml: phase 4 module type S = sig … end parser (+3 tests, 389 total)

module type S = sig DECLS end is parsed-and-discarded — sig..end
balanced skipping in parse-decl-module-type. AST (:module-type-def
NAME). Runtime no-op (signatures are type-level only).

Allows real OCaml programs with module type decls to parse and run
without stripping the sig blocks.

lib/ocaml/test.sh

@@ -956,6 +956,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 2603)
 (eval "(ocaml-run \"match { x = 5; y = 2 } with | { x = 1; y = y } -> y | _ -> 0\")")
 
+;; ── module type S = sig … end ─────────────────────────────────
+(epoch 2700)
+(eval "(ocaml-parse-program \"module type S = sig val x : int val f : int -> int end\")")
+(epoch 2701)
+(eval "(ocaml-run-program \"module type S = sig val x : int end ;; module M = struct let x = 42 end ;; M.x\")")
+(epoch 2702)
+(eval "(ocaml-parse-program \"module type EMPTY = sig end\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1512,6 +1520,11 @@ check 2601 "match record name+age"        '30'
 check 2602 "match record literal x=1"     '2'
 check 2603 "match record literal fail"    '0'
 
+# ── module type S = sig … end ──────────────────────────────────
+check 2700 "module type S parses"         '("module-type-def" "S")'
+check 2701 "module type then module"      '42'
+check 2702 "module type EMPTY"            '("module-type-def" "EMPTY")'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"