Lands the first walking slice of the type checker. `ck_types` gains the
fail-soft `Ty::Unknown` sentinel (absorbing composites, one short-circuit
in `assignable`), the `TypeRef → Ty` lift, the eager `lift_signatures`
pre-pass, and the bidirectional `TypeckResults` walker that emits the
four new E07xx codes:

- E0701 — argument-count mismatch at a call site
- E0702 — argument-type mismatch
- E0703 — trailing-expression vs. declared return
- E0704 — non-function callee

`ck_syntax` adds the minimal projections TY2 needs: `TypeRef<'a>`
(over `TypeExpr` / `UnionType`), plus `Param::ty`, `FnDecl::return_type`,
and `LetStmt::ty`. `ck_cktest` wires the type-check pass into the
conformance pipeline via a new `typecheck_pages` step.

Gating fixtures under `tests/types/`: let_infers_int,
trailing_expr_unit_mismatch, arg_type_mismatch, arity_mismatch,
unknown_callee, clean_body. Two parser fixtures gained `-> Int` to stay
type-clean.

Const annotations, op signatures, and actor slots remain deferred per
the spec's "what's open"; the body walker treats absent entries as
`Unknown`, keeping the cascade silent until later slices land.

@@ -0,0 +54,4 @@
+
+    /// The type recorded for a local binder, if any.
+    #[must_use]
+    pub fn type_of_local(&self, id: LocalId) -> Option<&Ty> {

@@ -393,0 +581,4 @@
+    }
+
+    #[test]
+    fn display_renders_each_variant_in_its_keyword_form() {