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/eval.sx

@@ -704,6 +704,7 @@
                           (set! result (merge result (dict mname mod-val))))))))
                 ((= tag "type-def") nil)
                 ((= tag "exception-def") nil)
+                ((= tag "module-type-def") nil)
                 ((= tag "open")
                   (let ((mod-val (ocaml-resolve-module-path (nth decl 1) env)))
                     (cond
@@ -884,6 +885,9 @@
                   ;; exception E [of T] — purely declarative; raise+match
                   ;; already work on tagged ctor values.
                   nil)
+                ((= tag "module-type-def")
+                  ;; module type S = sig … end — no-op at runtime.
+                  nil)
                 ((or (= tag "open") (= tag "include"))
                   ;; open M / include M — bring M's bindings into scope.
                   (let ((mod-val (ocaml-resolve-module-path (nth decl 1) env)))