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/sim/test/room.test.ts

@@ -458,7 +458,7 @@ describe("a simulation room's browser and web server windows", () => {
       [server, 'webServer', 'setVirtualHost', { row: 0, column: 1, value: 2 }, 'badArgs'],
       [server, 'webServer', 'restart', {}, 'unknownAction'],
       [notebook, 'webServer', 'setActive', { active: true }, 'unknownApp'],
-      [server, 'echoServer', 'setActive', { active: true }, 'unknownAction'],
+      [server, 'gnutella', 'join', {}, 'unknownAction'],
     ];
     for (const [target, app, action, args, code] of cases) {
       send({ kind: 'input', node: target, app, action, args, reqId: `${app}.${action}.${JSON.stringify(args)}` });