Add UDP socket client/server multiplayer protocol

Implements the multiplayer transport layer from ROADMAP.md: UUID-based
client identity, a reliable-packet channel over UDP, handshake/ack/
ping/disconnect/player-joined/player-left/player-state packet types,
and split online (networked) vs offline (in-process singleplayer)
server modes wired into the engine's update/dispose loop.

test/network/test_network.c

@@ -0,0 +1,62 @@
+/**
+ * Copyright (c) 2026 Dominic Masters
+ *
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#include "dusktest.h"
+#include "network/network.h"
+#include "network/networkinfo.h"
+#include "util/memory.h"
+
+typedef struct {
+  bool_t connected;
+  bool_t failed;
+} networkevents_t;
+
+static void onConnected(void *user) {
+  ((networkevents_t *)user)->connected = true;
+}
+
+static void onFailed(errorret_t error, void *user) {
+  errorCatch(error);
+  ((networkevents_t *)user)->failed = true;
+}
+
+static void onDisconnect(errorret_t error, void *user) {
+  errorCatch(error);
+}
+
+// Regression test: networkIsConnected() reports physical connectivity and
+// is NOT the same thing as NETWORK.state -- networkGetInfo() asserts on
+// the latter. Code must gate networkGetInfo() on NETWORK.state, reached
+// only via networkRequestConnection(), not on networkIsConnected() alone.
+static void test_request_connection_enables_get_info(void **state) {
+  assert_true(errorIsOk(networkInit()));
+
+  networkevents_t events;
+  memoryZero(&events, sizeof(events));
+
+  // Resolves synchronously on Linux (no networkPlatformRequestConnection).
+  networkRequestConnection(onConnected, onFailed, onDisconnect, &events);
+  assert_true(events.connected || events.failed);
+
+  if(events.connected) {
+    assert_int_equal(NETWORK.state, NETWORK_STATE_CONNECTED);
+    const networkinfo_t info = networkGetInfo(); // must not assert/crash
+    (void)info;
+  } else {
+    assert_int_equal(NETWORK.state, NETWORK_STATE_DISCONNECTED);
+  }
+
+  assert_true(errorIsOk(networkDispose()));
+  assert_int_equal(memoryGetAllocatedCount(), 0);
+}
+
+int main(void) {
+  const struct CMUnitTest tests[] = {
+    cmocka_unit_test(test_request_connection_enables_get_info),
+  };
+  return cmocka_run_group_tests(tests, NULL, NULL);
+}