feat(rpc): transport, server, client, http, mounting, docs (Tasks 5-11)

Brings the uncommitted body of work under version control so it cannot be
lost. Gates are green: 152 tests pass across rpc/router/dev-server,
typecheck, lint, format and check:public-api all clean.

NOT YET REVIEWED. None of Tasks 5-11 has had an independent task review, and
Task 4's second fix round was never re-reviewed either.

Known gaps against the plan, recorded here rather than discovered later:
- packages/rpc/test/{transport,server,client}.test.ts are ABSENT. The plan
  required a test file for each. server.ts holds the fail-closed identity and
  permission checks and currently has no direct coverage at all.
- rpc-endpoint.test.ts has 3 tests where the plan specified 9. Missing:
  unknown service, non-POST, malformed body, non-rpc passthrough, and the
  isInternalCaller sweep. This is the task where a reachable
  /__wrnexus/rpc/* makes every permission check in the workspace bypassable.
- http.test.ts has 3 of 7; integration.test.ts 2 of 3;
  services-discovery.test.ts 1 of 4.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/dev-server/src/gateway.ts

@@ -15,6 +15,9 @@ import { dirname, join, resolve } from "node:path";
 import { fileURLToPath } from "node:url";
 import { RESTART_EXIT_CODE } from "./restart.ts";
 
+const RPC_PATH_PREFIX = "/__wrnexus/rpc";
+const RPC_INTERNAL_HEADER = "x-wrnexus-internal";
+
 export type GatewayForwardAuth = (
   | {
       url: string;
@@ -432,6 +435,13 @@ export function gatewayProxyHeaders(
   return headers;
 }
 
+/** Remove headers that only a direct workspace-to-app request may supply. */
+export function stripUntrustedInternalHeaders(headers: Headers): Headers {
+  const sanitized = new Headers(headers);
+  sanitized.delete(RPC_INTERNAL_HEADER);
+  return sanitized;
+}
+
 /** Boot every app as a child process, then route by Host on one gateway port. */
 export async function startGateway(opts: GatewayOptions): Promise<RunningGateway> {
   const port = opts.port ?? 3000;
@@ -615,6 +625,10 @@ export async function startGateway(opts: GatewayOptions): Promise<RunningGateway
           });
         }
 
+        if (url.pathname === RPC_PATH_PREFIX || url.pathname.startsWith(`${RPC_PATH_PREFIX}/`)) {
+          return new Response("Not found", { status: 404 });
+        }
+
         // Edge rate limit (global, by client IP).
         if (rateLimit && !rateLimit(ip, now())) {
           return new Response("Too Many Requests", {
@@ -665,7 +679,9 @@ export async function startGateway(opts: GatewayOptions): Promise<RunningGateway
         }
 
         // HTTP → reverse-proxy to the app, preserving method/headers/body.
-        const headers = gatewayProxyHeaders(req, url, ip, forwardedHeaders);
+        const headers = stripUntrustedInternalHeaders(
+          gatewayProxyHeaders(req, url, ip, forwardedHeaders),
+        );
         const body =
           req.method === "GET" || req.method === "HEAD" ? undefined : await req.arrayBuffer();
         let res: Response;