plans: tick Phase 13 class-defaults test file (13/13, plan ≥10)

plans/haskell-completeness.md

@@ -240,7 +240,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   `zero :: a` aren't dispatchable in our 1-arg type-driven scheme; tests
   derive zero via `(mySub x x)` instead. signum tests skipped — needs
   `signum` literal handling that's too tied to Phase 10's int/float design._
-- [ ] Tests in `lib/haskell/tests/class-defaults.sx` (≥ 10 tests).
+- [x] Tests in `lib/haskell/tests/class-defaults.sx` (13/13, plan ≥10).
 - [ ] Conformance programs:
   - `shapes.hs` — `class Area a` with a default `perimeter`; two instances
     using `where`-local helpers.