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/rpc/README.md

@@ -0,0 +1,44 @@
+# `@wrnexus/rpc`
+
+Define a service contract in a shared workspace package, then import that same contract from the caller and callee.
+
+```ts
+import {
+  defineService,
+  implement,
+  inProcessTransport,
+  procedure,
+  serviceClient,
+} from "@wrnexus/rpc";
+import { v } from "@wrnexus/validation";
+
+const greeter = defineService({
+  name: "greeter",
+  procedures: {
+    greet: procedure
+      .input(v.object({ name: v.string() }))
+      .output<{ message: string }>()
+      .build(),
+  },
+});
+
+const service = implement(
+  greeter,
+  { greet: async ({ name }) => ({ message: `Hello, ${name}` }) },
+  { selfApp: "greeter" },
+);
+
+const client = serviceClient(greeter, {
+  app: "greeter",
+  transport: inProcessTransport({
+    "greeter/greet": (input, identity) => service.invoke("greet", input, identity),
+  }),
+});
+await client.greet({ name: "Ada" });
+```
+
+Service files default-export `implement(...)` from `app/services`. The development server mounts them under the private `/__wrnexus/rpc` prefix.
+
+Pass `{ as: ctx }` to `serviceClient` to propagate the subject. The signed token contains only subject and tenant identifiers; permissions are always checked by the callee. Set `WRNEXUS_RPC_SECRET` in every app, use at least 32 characters, and never reuse the session secret.
+
+Calls time out by default. Retrying is intentionally deferred; when introduced, only procedures marked `.idempotent()` may be retried.