feat(rpc): add target policy, protocol schemas, and tagged errors

- Add normalizeLoopbackTarget() with loopback-only URL validation
- Add JSON-RPC response decoder with schema validation
- Define 8 tagged errors using Data.TaggedError
- 20 tests covering target policy and protocol decoding
- Re-export all public types from index.ts

web/packages/rpc/src/index.ts

@@ -0,0 +1,21 @@
+export {
+  InvalidTargetError,
+  UnknownOperationError,
+  UpstreamConnectionError,
+  UpstreamTimeoutError,
+  UpstreamResponseTooLargeError,
+  RpcProtocolError,
+  RpcRemoteError,
+  RpcDecodeError,
+} from "./errors.js";
+
+export { normalizeLoopbackTarget } from "./target-policy.js";
+
+export { decodeRpcResponse } from "./protocol.js";
+
+export type {
+  JsonRpcRequest,
+  JsonRpcResponse,
+  JsonRpcSuccess,
+  JsonRpcFailure,
+} from "./protocol.js";