fix(serde): add schema::default_value to skip_if_default

`skip_if_default<T>` omits fields during serialization when they equal
the default value, but did not declare them as optional during
deserialization. After #95 added required field checking, this caused
LSP types using `optional_bool` (= `skip_if_default<bool>`) to reject
valid messages when clients omit proposed/newer capability fields.

Fix: add `schema::default_value` to `skip_if_default`'s annotation list
so the field is recognized as optional during deserialization. This keeps
serialization and deserialization semantics orthogonal — `skip_if` alone
remains serialization-only, `default_value` alone is deserialization-only,
and `skip_if_default` explicitly opts into both.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 16bit-ykiko

include/eventide/serde/serde/attrs.h

@@ -42,6 +42,6 @@ template <typename T>
 using skip_if_empty = annotation<T, behavior::skip_if<pred::empty>>;
 
 template <typename T>
-using skip_if_default = annotation<T, behavior::skip_if<pred::default_value>>;
+using skip_if_default = annotation<T, behavior::skip_if<pred::default_value>, schema::default_value>;
 
 }  // namespace eventide::serde