sx: step 5 — OCaml AdtValue + define-type + match

Native algebraic data type representation in the OCaml SX evaluator.
Replaces the dict-based shim that simulated ADT values via tagged dicts.

- sx_types.ml: add AdtValue variant + adt_value record (av_type, av_ctor,
  av_fields). type_of returns the type name (e.g. "Maybe"); inspect renders
  as a constructor call (e.g. "(Just 42)" or "(Nothing)").
- sx_runtime.ml: get_val handles AdtValue with :_adt/:_type/:_ctor/:_fields
  keys for back-compat with spec-level match-pattern code.
- sx_primitives.ml: dict? returns true for AdtValue (so existing match
  dispatch keeps working); new adt? predicate distinguishes ADT values.
- sx_ref.ml: sf_define_type now constructs AdtValue instead of Dict.
  Predicates (Name?, Ctor?) and accessors (Ctor-field) match on AdtValue
  with proper type/ctor name and field index checks.
- spec/tests/test-adt.sx: 3 new tests covering type-of, adt?, and inspect.

Tests: 4532 passed (was 4529 + 3 new), 1339 failed (unchanged baseline).
All 43 ADT tests pass on the native representation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hosts/ocaml/lib/sx_runtime.ml

@@ -209,6 +209,13 @@ let get_val container key =
      | _ -> Nil)
   | Dict d, String k -> dict_get d k
   | Dict d, Keyword k -> dict_get d k
+  | AdtValue a, String k | AdtValue a, Keyword k ->
+    (match k with
+     | "_adt"    -> Bool true
+     | "_type"   -> String a.av_type
+     | "_ctor"   -> String a.av_ctor
+     | "_fields" -> List (Array.to_list a.av_fields)
+     | _         -> Nil)
   | (List l | ListRef { contents = l }), Number n ->
     (try List.nth l (int_of_float n) with _ -> Nil)
   | (List l | ListRef { contents = l }), Integer n ->