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/test.sh

@@ -1156,6 +1156,12 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4502)
 (eval "(ocaml-run \"try raise (E 5) with | E n when n > 100 -> n | E n -> n + 1000\")")
 
+;; ── type aliases ──────────────────────────────────────────────
+(epoch 4600)
+(eval "(ocaml-parse-program \"type t = int\")")
+(epoch 4601)
+(eval "(ocaml-run-program \"type t = int;; 42\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1831,6 +1837,10 @@ check 4500 "try when guard fires"         '5'
 check 4501 "try when guard skips"         '0'
 check 4502 "try when fall through"        '1005'
 
+# ── type aliases ───────────────────────────────────────────────
+check 4600 "type t = int parses"          '("type-alias" "t" ())'
+check 4601 "type alias decl + use"        '42'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"