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>

plans/sx-improvements.md

@@ -189,7 +189,7 @@ these when operands are known numbers/lists.
 | 2 — letrec+resume | [x] | e80e655b |
 | 3 — tokenizer :end/:line | [x] | 023bc2d8 |
 | 4 — parser spans complete | [x] | b7ad5152 (subsumed by 023bc2d8) |
-| 5 — OCaml AdtValue + define-type + match | [ ] | — |
+| 5 — OCaml AdtValue + define-type + match | [x] | (pending) |
 | 6 — JS AdtValue + define-type + match | [ ] | — |
 | 7 — nested patterns | [ ] | — |
 | 8 — exhaustiveness warnings | [ ] | — |