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-codegen/src/lib.rs

@@ -765,6 +765,13 @@ impl<'a> Emitter<'a> {
                     // logical type. Heap boxes are allocated ad hoc via
                     // GC_malloc (Boehm conservative collector, Iter 14f).
                 }
+                // Iter 22b.1: class/instance defs do not emit IR yet.
+                // 22b.3 monomorphisation will rewrite class-method
+                // calls into calls against synthesised monomorphic
+                // FnDefs; once that pass runs, class/instance bodies
+                // never reach the emit path on their own — they only
+                // appear inlined into the synthesised fns.
+                Def::Class(_) | Def::Instance(_) => {}
             }
         }
         // Drain the monomorphisation queue. Specialised fns may