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/src/runtime/ipv4.ts

@@ -0,0 +1,48 @@
+// Ported from filius-ipad (GPLv2/GPLv3):
+//   State/TopologyNetworkRuntimeEngine.swift (parseIPv4, sameSubnet)
+//   State/TopologyEditorState.swift (TopologyRuntimeManualRoute.parseIPv4)
+//
+// Two parsers exist on purpose, as in the Swift source: the runtime reads an
+// octet the way Swift's `UInt8(String)` does, the route editor the way Java
+// FILIUS validates its table cells (see routeTable.ts).
+
+/**
+ * Swift `UInt8(segment)`: an optional sign, then decimal digits, value 0…255.
+ * Leading zeros are allowed ("007" is 7), whitespace is not.
+ */
+function parseSwiftOctet(segment: string): number | undefined {
+  if (!/^[+-]?[0-9]+$/.test(segment)) return undefined;
+  const value = Number(segment);
+  if (!Number.isInteger(value) || value < 0 || value > 255) return undefined;
+  return Object.is(value, -0) ? 0 : value;
+}
+
+/** Dotted quad to an unsigned 32-bit number, or undefined. */
+export function parseIPv4(value: string): number | undefined {
+  const segments = value.split('.');
+  if (segments.length !== 4) return undefined;
+  let address = 0;
+  for (const segment of segments) {
+    const octet = parseSwiftOctet(segment);
+    if (octet === undefined) return undefined;
+    address = ((address << 8) | octet) >>> 0;
+  }
+  return address;
+}
+
+export function formatIPv4(address: number): string {
+  return [24, 16, 8, 0].map((shift) => String((address >>> shift) & 0xff)).join('.');
+}
+
+export function sameSubnet(lhs: string, rhs: string, mask: string): boolean {
+  const left = parseIPv4(lhs);
+  const right = parseIPv4(rhs);
+  const netmask = parseIPv4(mask);
+  if (left === undefined || right === undefined || netmask === undefined) return false;
+  return ((left & netmask) >>> 0) === ((right & netmask) >>> 0);
+}
+
+/** `address & mask` as an unsigned number. */
+export function maskAddress(address: number, mask: number): number {
+  return (address & mask) >>> 0;
+}