Open the type-checker stage in ck_types. TY1 is pure judgment
infrastructure, consumed by every later slice:

- the `Ty` enum: primitives (Int default; widths provisional), the
  String/EmptyString split, nominal records (TypeId + invariant args),
  normalized unions, type variables, Fn types, Nothing, Unit;
- `Ty::union`: the normalizing constructor (flatten, absorb Nothing,
  dedupe, sort, collapse singleton/empty) so two spellings of one union
  compare equal in one step;
- `assignable(from, to)`: the whole of CkLang's "subtyping" — four cases
  (identity / tag-membership-into-union / Nothing / the String-EmptyString
  split), no lattice, no variance, no width or numeric coercion.

TypeVarId is checker-allocated, so it lives here, not in the resolver's
interner. TY1 emits no diagnostic; the E07xx block opens when TY2's
bidirectional checker fires the first argument-type mismatch.

17 unit tests cover the four cases, invariance, the payload-free-variant
rule, and the five normalization laws. Gates: build 0, clippy 0
(deny all+pedantic), workspace 652 tests pass.

@@ -5,0 +292,4 @@
+    fn generics_are_invariant() {
+        // List<Cat> is unrelated to List<Animal> even though Cat ∈ Animal.
+        let animal = Ty::union([cat(), dog()]);
+        assert!(!assignable(&list(cat()), &list(animal.clone())));