haskell: Phase 13 conformance — shapes.hs (5/5), Phase 13 complete

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/haskell-completeness.md

@@ -241,7 +241,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   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._
 - [x] Tests in `lib/haskell/tests/class-defaults.sx` (13/13, plan ≥10).
-- [ ] Conformance programs:
+- [x] Conformance programs:
   - `shapes.hs` — `class Area a` with a default `perimeter`; two instances
     using `where`-local helpers.
 
@@ -313,6 +313,13 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 13 conformance: shapes.hs (5/5) → Phase 13 complete:
+- `class Shape` with a default `perimeter` (using a where-clause inside the
+  default body), two instances `Square` / `Rect` — Square overrides
+  `perimeter`, Rect's `perimeter` uses a where-bound `peri`. 5/5 across
+  area, perimeter (override), perimeter-via-where, sum. Phase 13 fully
+  complete.
+
 **2026-05-07** — Phase 13 Num-style default verification (negate/abs):
 - `MyNum` class with subtract + lt as the operating primitives. Defaults for
   `myNegate x` and `myAbs x` derive zero via `mySub x x`. Zero-arity class