iter 22b.1.1: ClassDef/InstanceDef AST + downstream match arms

Adds Def::Class(ClassDef) and Def::Instance(InstanceDef) variants per
Decision 11 §"Form-A schema". All optional fields (superclass, doc,
default body) carry skip_serializing_if so canonical-JSON bytes of
pre-22b fixtures stay bit-identical.

Downstream match-on-Def sites are extended with explicit Class/Instance
arms. Behaviour for 22b.1 is placeholder (skip in typecheck/codegen,
one-line summary in pretty/manifest, placeholder marker in prose/print)
with TODO comments naming the deferred sub-iters (22b.2 typecheck,
22b.3 codegen, 22b.4 prose-projection arms).

crates/ailang-check/src/linearity.rs

@@ -205,6 +205,11 @@ pub(crate) fn check_module(m: &Module) -> Vec<Diagnostic> {
                     ctors.insert(name.clone(), fields.clone());
                 }
             }
+            // Iter 22b.1: class/instance defs are skipped here. Once
+            // 22b.3 monomorphisation materialises class methods as
+            // ordinary `FnDef`s, the lift's globals map will pick
+            // them up automatically.
+            Def::Class(_) | Def::Instance(_) => {}
         }
     }