feat(protocol): add codec module with encode/decode and envelope builders

- Add encodeBuiltin/decodeBuiltin for builtin message wire format
- Add parseRuleMessage/parseRewrittenRuleMessage for rule dispatch
- Add type-safe envelope builders for all builtin message types
- Export codec module from index.ts

src/index.ts

@@ -1 +1,2 @@
 export * from "./types.js";
+export * from "./codec.js";