feat(interface): add authoring macros and codecs

Add the wip-interface facade and proc-macro implementation for typed interface descriptors, DTO value codecs, validated dispatch, metadata extraction, and span-aware compile failures.

crates/wip-interface/tests/ui/mixed_enum.rs

@@ -0,0 +1,9 @@
+use wip_interface::WipType;
+
+#[derive(WipType)]
+enum MixedEnum {
+    Empty,
+    Payload(String),
+}
+
+fn main() {}