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/core/src/runtime/engine.ts

@@ -2799,11 +2799,21 @@ export class NetworkRuntimeEngine {
     }
   }
 
+  /**
+   * A data segment (TCPSocket's receive loop): new data goes into the
+   * message, a push ends it. An empty message is one empty segment with the
+   * push (TCPSocket.erstelleSegmente makes at least one; an empty segment
+   * comes here only with it). Java ends a message with every push, also with
+   * a repeated segment's (an ACK lost: an empty message more); here only
+   * with a new one's — but an empty segment repeated cannot be told from a
+   * new one by its number, which it does not count on, and is an empty
+   * message again (named).
+   */
   private receiveTcpPayload(socketId: Id, segment: TcpSegment): void {
     const session = this.stateValue.tcpSessionsById.get(socketId);
     if (session === undefined) return;
     const nextRemoteSequence = nextTcpSequenceNumber(segment);
-    if (nextRemoteSequence > session.remoteSequenceNumber) {
+    if (nextRemoteSequence > session.remoteSequenceNumber || segment.payload.length === 0) {
       session.remoteSequenceNumber = nextRemoteSequence;
       session.receiveAssembly = concatBytes(session.receiveAssembly, segment.payload);
       if ((segment.flags & TCP_FLAG.push) !== 0) {