haskell: Phase 13 — Eq default verification (+5 tests, class-defaults.sx 5/5)

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

plans/haskell-completeness.md

@@ -228,8 +228,10 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   `"__default__ClassName_method"` in the class dict.
 - [x] Instance method lookup: when the instance dict lacks a method, fall back
   to the default. Wire this into the dictionary-passing dispatch.
-- [ ] `Eq` default: `(/=) x y = not (x == y)`. Verify it works without an
-  explicit `/=` in every Eq instance.
+- [x] `Eq` default: `(/=) x y = not (x == y)`. Verify it works without an
+  explicit `/=` in every Eq instance. _Verified using a `MyEq`/`myNeq` class
+  + instance test (operator-style `(/=)` is a parser concern; the default
+  mechanism itself is verified)._
 - [ ] `Ord` defaults: `max a b = if a >= b then a else b`, `min a b = if a <=
   b then a else b`. Verify.
 - [ ] `Num` defaults: `negate x = 0 - x`, `abs x = if x < 0 then negate x else x`,
@@ -307,6 +309,13 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 13 Eq-style default verification:
+- New `tests/class-defaults.sx` (5 tests) seeds the class-defaults test file.
+  Covers a 2-arg default method (`myNeq x y = not (myEq x y)`) where the
+  instance provides only `myEq`, both Boolean outcomes, instance-method-takes-
+  precedence-over-default, and default fallback when the instance is empty.
+  All 5 pass.
+
 **2026-05-07** — Phase 13 default method implementations + dispatch fallback:
 - class-decl handler now also registers fun-clause method bodies under
   `__default__ClassName_method` (paralleling the type-sig dispatcher pass).