feat(core): event queue, Ethernet, switch, ARP, IPv4, ICMP, routing and firewall

First slice of the TypeScript port of the filius-ipad runtime engine,
with 29 tests ported from the Swift suite. Parts not ported yet throw
NotPortedError instead of guessing.

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

packages/core/test/fixtures.ts

@@ -0,0 +1,50 @@
+// Test fixtures, ported from the private helpers of
+// filius-ipad ios/FiliusPadTests/TopologyNetworkRuntimeEngineTests.swift.
+
+import {
+  deviceConfiguration,
+  NetworkRuntimeEngine,
+  portSnapshot,
+  topologySnapshot,
+  type DeviceConfiguration,
+  type Id,
+  type LinkSnapshot,
+  type NodeSnapshot,
+  type TopologySnapshotInput,
+} from '../src/index.js';
+
+let linkCounter = 0;
+
+/** Swift uses a fresh `UUID()` per link; a counter keeps runs reproducible. */
+export function link(sourceNodeId: Id, sourcePortId: Id, targetNodeId: Id, targetPortId: Id): LinkSnapshot {
+  linkCounter += 1;
+  return {
+    id: `ffffffff-0000-0000-0000-${linkCounter.toString(16).padStart(12, '0')}`,
+    sourceNodeId,
+    sourcePortId,
+    targetNodeId,
+    targetPortId,
+  };
+}
+
+export function pc(nodeId: Id, portId: Id): NodeSnapshot {
+  return { id: nodeId, kind: 'pc', ports: [portSnapshot(portId, 'eth0')] };
+}
+
+export function device(ipAddress: string, options: { mask?: string; gateway?: string } = {}): DeviceConfiguration {
+  return deviceConfiguration(ipAddress, options.mask ?? '255.255.255.0', options.gateway ?? '');
+}
+
+export function startedEngine(
+  input: TopologySnapshotInput,
+  seed: number = 14,
+): NetworkRuntimeEngine {
+  const engine = new NetworkRuntimeEngine(seed);
+  engine.handle({ type: 'start', snapshot: topologySnapshot(input), seed, initialTimeMilliseconds: 0 });
+  return engine;
+}
+
+/** `00000000-0000-0000-0000-` followed by twelve hex digits. */
+export function uuid(suffix: number): Id {
+  return `00000000-0000-0000-0000-${suffix.toString(16).toUpperCase().padStart(12, '0')}`;
+}