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/target-policy.ts

@@ -0,0 +1,57 @@
+import { InvalidTargetError } from "./errors.js";
+
+export { InvalidTargetError };
+
+const ALLOWED_HOSTNAMES = new Set(["127.0.0.1", "localhost", "[::1]"]);
+
+/**
+ * Normalize and validate a loopback RPC target URL.
+ *
+ * Only `http://127.0.0.1`, `http://localhost`, and `http://[::1]` are accepted.
+ * DNS names other than the literal `localhost` are rejected to avoid
+ * DNS-rebinding ambiguity — a hostname that resolves to a non-loopback
+ * address at resolution time would bypass this check.
+ */
+export function normalizeLoopbackTarget(raw: string): string {
+  let url: URL;
+  try {
+    url = new URL(raw);
+  } catch {
+    throw new InvalidTargetError({ message: "invalid URL" });
+  }
+
+  if (url.protocol !== "http:") {
+    throw new InvalidTargetError({ message: "only http: protocol is allowed" });
+  }
+
+  if (!ALLOWED_HOSTNAMES.has(url.hostname)) {
+    throw new InvalidTargetError({
+      message: `hostname "${url.hostname}" is not an allowed loopback address`,
+    });
+  }
+
+  if (url.username !== "" || url.password !== "") {
+    throw new InvalidTargetError({ message: "credentials in URL are not allowed" });
+  }
+
+  if (url.port === "") {
+    throw new InvalidTargetError({ message: "explicit port is required" });
+  }
+
+  const port = Number(url.port);
+  if (!Number.isInteger(port) || port < 1 || port > 65535) {
+    throw new InvalidTargetError({
+      message: `port ${url.port} is outside valid range 1..65535`,
+    });
+  }
+
+  if (url.search !== "") {
+    throw new InvalidTargetError({ message: "query string is not allowed" });
+  }
+
+  if (url.hash !== "") {
+    throw new InvalidTargetError({ message: "fragment is not allowed" });
+  }
+
+  return url.toString();
+}