docs: 複数コードベースのホスト振り分け例を追加 (host routing sample)

samples/multiple-codebases/README.md

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+# Multiple Codebases Example
+
+This example runs two applications with separate Worker scripts in one workerd
+process. Both applications share a listening socket. A router Worker selects an
+application by the request URL's hostname and forwards the request through a
+service binding. Unrecognized hostnames receive a 404 response.
+
+Use this pattern for codebases you trust, such as multiple applications belonging
+to the same organization. Host-based routing does not provide the security
+isolation needed to run arbitrary, potentially malicious tenant code.
+
+## Run
+
+From this directory, using a workerd binary:
+
+```sh
+workerd serve config.capnp
+```
+
+Or from the repository root, using Bazel:
+
+```sh
+bazel run //src/workerd/server:workerd -- serve "$PWD/samples/multiple-codebases/config.capnp"
+```
+
+## Try Both Applications
+
+The example listens on `127.0.0.1:8080`. Set the `Host` header to select an
+application without configuring local DNS:
+
+```sh
+curl -H 'Host: one.example.test' 'http://127.0.0.1:8080/hello?name=one'
+# App one: http://one.example.test/hello?name=one
+
+curl -H 'Host: two.example.test' 'http://127.0.0.1:8080/hello?name=two'
+# App two: http://two.example.test/hello?name=two
+
+curl -i -H 'Host: unknown.example.test' http://127.0.0.1:8080/
+# HTTP/1.1 404 Not Found
+```
+
+Each application's service can have its own modules, bindings, and compatibility
+date. To add an application, configure its service, give the router a binding to
+that service, and add its hostname to the router. The original request, including
+its path, query string, method, and body, is passed to the selected application.

samples/multiple-codebases/app-one.js

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+// Copyright (c) 2026 Cloudflare, Inc.
+// Licensed under the Apache 2.0 license found in the LICENSE file or at:
+//     https://opensource.org/licenses/Apache-2.0
+
+export default {
+  fetch(request) {
+    return new Response(`App one: ${request.url}\n`);
+  },
+};

samples/multiple-codebases/app-two.js

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+// Copyright (c) 2026 Cloudflare, Inc.
+// Licensed under the Apache 2.0 license found in the LICENSE file or at:
+//     https://opensource.org/licenses/Apache-2.0
+
+export default {
+  fetch(request) {
+    return new Response(`App two: ${request.url}\n`);
+  },
+};

samples/multiple-codebases/config.capnp

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+using Workerd = import "/workerd/workerd.capnp";
+
+const multipleCodebasesExample :Workerd.Config = (
+  services = [
+    (name = "router", worker = (
+      modules = [(name = "router", esModule = embed "router.js")],
+      compatibilityDate = "2023-02-28",
+      bindings = [
+        (name = "APP_ONE", service = "app-one"),
+        (name = "APP_TWO", service = "app-two"),
+      ],
+    )),
+    (name = "app-one", worker = (
+      modules = [(name = "app", esModule = embed "app-one.js")],
+      compatibilityDate = "2023-02-28",
+    )),
+    (name = "app-two", worker = (
+      modules = [(name = "app", esModule = embed "app-two.js")],
+      compatibilityDate = "2023-02-28",
+    )),
+  ],
+  sockets = [
+    (name = "http", address = "127.0.0.1:8080", http = (), service = "router"),
+  ],
+);

samples/multiple-codebases/router.js

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+// Copyright (c) 2026 Cloudflare, Inc.
+// Licensed under the Apache 2.0 license found in the LICENSE file or at:
+//     https://opensource.org/licenses/Apache-2.0
+
+export default {
+  fetch(request, env) {
+    switch (new URL(request.url).hostname) {
+      case 'one.example.test':
+        return env.APP_ONE.fetch(request);
+      case 'two.example.test':
+        return env.APP_TWO.fetch(request);
+      default:
+        return new Response('Unknown host\n', { status: 404 });
+    }
+  },
+};