ocaml: phase 1+5.1 type aliases + poly_stack baseline (+3 tests, 469 / 19 baseline)

Parser: in parse-decl-type, dispatch on the post-= token:
  '|' or Ctor   -> sum type
  '{'           -> record type
  otherwise     -> type alias (skip to boundary)
AST (:type-alias NAME PARAMS) with body discarded. Runtime no-op since
SX has no nominal types.

poly_stack.ml baseline exercises:
  module type ELEMENT = sig type t val show : t -> string end
  module IntElem = struct type t = int let show x = ... end
  module Make (E : ELEMENT) = struct ... use E.show ... end
  module IntStack = Make(IntElem)

Demonstrates the substrate handles signature decls + abstract types +
functor parameter with sig constraint.

lib/ocaml/eval.sx

@@ -807,6 +807,7 @@
                           (set! result (merge result (dict mname mod-val))))))))
                 ((= tag "type-def") nil)
                 ((= tag "type-def-record") nil)
+                ((= tag "type-alias") nil)
                 ((= tag "exception-def") nil)
                 ((= tag "module-type-def") nil)
                 ((= tag "open")
@@ -993,6 +994,9 @@
                   ;; type r = { x : T; y : T } — runtime no-op; records
                   ;; are already dynamic dicts.
                   nil)
+                ((= tag "type-alias")
+                  ;; type t = SomeType — runtime no-op (no nominal types).
+                  nil)
                 ((= tag "module-type-def")
                   ;; module type S = sig … end — no-op at runtime.
                   nil)