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-surface/src/print.rs

@@ -94,6 +94,25 @@ fn write_def(out: &mut String, def: &Def, level: usize) {
         Def::Type(td) => write_type_def(out, td, level),
         Def::Fn(fd) => write_fn_def(out, fd, level),
         Def::Const(cd) => write_const_def(out, cd, level),
+        // Iter 22b.1: surface (Form-B) printer arms for ClassDef /
+        // InstanceDef are deferred to 22b.4 (along with the parse-side
+        // arms in `surface/src/parse.rs`). For 22b.1 we emit a
+        // single-line placeholder so a module containing these defs
+        // still round-trips through `ail render` in print-only mode.
+        Def::Class(c) => {
+            indent(out, level);
+            out.push_str("(class ");
+            out.push_str(&c.name);
+            out.push(' ');
+            out.push_str(&c.param);
+            out.push(')');
+        }
+        Def::Instance(i) => {
+            indent(out, level);
+            out.push_str("(instance ");
+            out.push_str(&i.class);
+            out.push(')');
+        }
     }
 }