ocaml: phase 3 exception declarations (+4 tests, 304 total)

exception NAME [of TYPE] parses to (:exception-def NAME [ARG-SRC]).
Runtime is a no-op: raise/match already work on tagged ctor values, so
'exception E of int;; try raise (E 5) with | E n -> n' end-to-end with
zero new eval logic.

lib/ocaml/eval.sx

@@ -633,6 +633,7 @@
                           (set! env (ocaml-env-extend env mname mod-val))
                           (set! result (merge result (dict mname mod-val))))))))
                 ((= tag "type-def") nil)
+                ((= tag "exception-def") nil)
                 ((= tag "open")
                   (let ((mod-val (ocaml-resolve-module-path (nth decl 1) env)))
                     (cond
@@ -809,6 +810,10 @@
                   ;; are dispatched by tag at eval/match time. Phase 5
                   ;; HM extensions will register ctor types here.
                   nil)
+                ((= tag "exception-def")
+                  ;; exception E [of T] — purely declarative; raise+match
+                  ;; already work on tagged ctor values.
+                  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)))