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-core/tests/spec_drift.rs

@@ -272,6 +272,14 @@ fn spec_mentions_every_def_kind() {
             Def::Fn(_) => "fn",
             Def::Const(_) => "const",
             Def::Type(_) => "type",
+            // Iter 22b.1: class/instance Def variants exist but are
+            // not yet anchored in the prose-spec block. Once 22b.4
+            // adds prose projection for them, the FORM_A_SPEC text
+            // gains `(class ` / `(instance ` anchors and this match
+            // will be exercised. For 22b.1 the exemplars list above
+            // does not produce these variants.
+            Def::Class(_) => "class",
+            Def::Instance(_) => "instance",
         };
         assert!(
             FORM_A_SPEC.contains(anchor),