ocaml: phase 3 type declarations (+5 tests, 300 total)

Parser: type [PARAMS] NAME = | Ctor [of T1 [* T2]*] | ...
- PARAMS: optional 'a or ('a, 'b) tyvar list
- AST: (:type-def NAME PARAMS CTORS) with each CTOR (NAME ARG-SOURCES)
- Argument types captured as raw source strings (treated opaquely at
  runtime since ctor dispatch is dynamic)

Runtime is a no-op — constructors and pattern matching already work
dynamically. Phase 5 will use these decls to register ctor types for
HM checking.

lib/ocaml/eval.sx

@@ -632,6 +632,7 @@
                         (begin
                           (set! env (ocaml-env-extend env mname mod-val))
                           (set! result (merge result (dict mname mod-val))))))))
+                ((= tag "type-def") nil)
                 ((= tag "open")
                   (let ((mod-val (ocaml-resolve-module-path (nth decl 1) env)))
                     (cond
@@ -803,6 +804,11 @@
                         (begin
                           (set! env (ocaml-env-extend env mname mod-val))
                           (set! last mod-val))))))
+                ((= tag "type-def")
+                  ;; type t = ... — purely declarative at runtime; ctors
+                  ;; are dispatched by tag at eval/match time. Phase 5
+                  ;; HM extensions will register ctor types here.
+                  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)))