feat(core): programs' threads, and FILIUS' echo server and generic client after Java, measured against Java

Programs run as generators that yield what they wait for (threads.ts); the
engine resumes them after every event. TCP connect, send, receive, accept
and name resolution exist as such steps; the blocking calls wrap them.
EchoRun records eight stories in Java FILIUS, frames, tables and each
program's window lines; echoJava.test.ts plays them card by card. Not
taken from Java: a connection without a route hangs for ever; a server
switched off leaves its connections' last FIN unanswered.

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

packages/fls/src/projectSnapshot.ts

@@ -4,9 +4,11 @@
 //
 // Only the cables count: a host's SSID connects nothing at run time, as in
 // Java (Swift joins a wireless host to the switch of its SSID; not taken).
-// A DNS server runs only when it is switched on (Java: DNSServer.aktiv).
+// A DNS server runs only when it is switched on (Java: DNSServer.aktiv); an
+// echo server is there either way, switched on or off (it can be switched at
+// run time).
 
-import { portSnapshot, topologySnapshot, type DnsServerConfiguration, type Id, type TopologySnapshot } from '@mdg-filius/core';
+import { portSnapshot, topologySnapshot, type DnsServerConfiguration, type EchoServerSnapshot, type Id, type TopologySnapshot } from '@mdg-filius/core';
 import type { ImportedProject } from './importProject.js';
 
 export function projectSnapshot(project: ImportedProject): TopologySnapshot {
@@ -14,6 +16,10 @@ export function projectSnapshot(project: ImportedProject): TopologySnapshot {
   for (const [nodeId, configuration] of project.dnsServerConfigurations) {
     if (project.activePrograms.get(nodeId)?.has('dnsServer') === true) dnsServers.set(nodeId, configuration);
   }
+  const echoServers = new Map<Id, EchoServerSnapshot>();
+  for (const [nodeId, configuration] of project.echoServerConfigurations) {
+    echoServers.set(nodeId, { ...configuration, isActive: project.activePrograms.get(nodeId)?.has('echoServer') === true });
+  }
   return topologySnapshot({
     nodes: project.graph.nodes.map((node) => ({
       id: node.id,
@@ -32,5 +38,7 @@ export function projectSnapshot(project: ImportedProject): TopologySnapshot {
     portForwardingRowsByNodeId: project.portForwardingRowsByNodeId,
     switchConfigurationsByNodeId: project.switchConfigurations,
     ipForwardingByNodeId: project.ipForwardingByNodeId,
+    echoServersByNodeId: echoServers,
+    simpleClientsByNodeId: project.simpleClientConfigurations,
   });
 }