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/private_trait.rs

@@ -0,0 +1,8 @@
+use wip_interface::{OperationResult, interface};
+
+#[interface]
+trait PrivateInterface {
+    fn load(&self) -> OperationResult<String>;
+}
+
+fn main() {}