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/compile_fail.rs

@@ -0,0 +1,5 @@
+#[test]
+fn rejects_unsupported_interface_and_dto_syntax() {
+    let tests = trybuild::TestCases::new();
+    tests.compile_fail("tests/ui/*.rs");
+}