feat: F4.6g1 the echo server and the generic client windows

Oracle EchoWindowRun uses Java's echo server and client windows as a
person does; SaveAfterRunRun echo saves what Java keeps after the run.
The room takes setActive with a port, connect/disconnect/send, sends
both views and writes the server's port and the client's target back.
The editor's windows show Java's buttons, fields and lines; a port that
is no port is marked and not handed over (Java's button throws).
Core fix: an empty message (one empty segment with the push, as Java's
erstelleSegmente makes) counted as a repeat and never arrived.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/protocol/src/echo.ts

@@ -0,0 +1,25 @@
+// What the echo server's and the generic client's windows show besides their
+// lines, as the room keeps them and every client shows them (plan §7.3),
+// after GUIApplicationServerBausteinWindow and
+// GUIApplicationClientBausteinWindow (measured with tools/java-oracle's
+// EchoWindowRun).
+
+/** The echo server: its button (running or not) and its port, which can be typed into only while it does not run. */
+export interface EchoServerView {
+  readonly active: boolean;
+  readonly port: number;
+}
+
+/** The generic client: "Verbinden" or "Trennen", and the server's address and port, which can be typed into only while it is not connected. */
+export interface SimpleClientView {
+  readonly connected: boolean;
+  readonly address: string;
+  readonly port: number;
+}
+
+/** A port as Java's windows take it (Integer.parseInt), and as a socket can have it: a whole number from 0 to 65535; undefined for any other text. */
+export function portOfText(text: string): number | undefined {
+  if (!/^\d+$/.test(text)) return undefined;
+  const port = Number(text);
+  return port <= 65_535 ? port : undefined;
+}

packages/protocol/src/index.ts

@@ -2,5 +2,6 @@
 export * from './host.js';
 export * from './sim.js';
 export * from './terminal.js';
+export * from './echo.js';
 export * from './mail.js';
 export * from './web.js';

packages/protocol/src/sim.ts

@@ -75,8 +75,10 @@ export type HostMessage =
   /**
    * What a program's window shows besides its lines (plan §7.3), whole: on
    * subscribing and when it changes; null while it has nothing (its program
-   * does not run, a browser that never ran). The browser's (BrowserView) and
-   * the web server's (WebServerView) so far.
+   * does not run, a browser that never ran): the browser's (BrowserView), the
+   * web server's (WebServerView), the e-mail program's and server's
+   * (EmailClientView, EmailServerView), the echo server's and the generic
+   * client's (EchoServerView, SimpleClientView).
    */
   | { readonly v: 1; readonly kind: 'view'; readonly node: string; readonly app: AppKind; readonly view: unknown }
   /**