feat: publish NIRO Gen5W research overview and reviewed source examples

tests/php/NiroMqttKeepaliveTest.php

@@ -0,0 +1,33 @@
+<?php
+declare(strict_types=1);
+
+require_once __DIR__ . '/../../backend/php/NiroMqttKeepalive.php';
+
+function assertMqttKeepalive(bool $condition, string $message): void
+{
+    if (!$condition) {
+        fwrite(STDERR, "NiroMqttKeepaliveTest failed: {$message}\n");
+        exit(1);
+    }
+}
+
+$keepalive = new NiroMqttKeepalive(100, 15);
+for ($now = 101; $now < 115; $now++) {
+    assertMqttKeepalive(!$keepalive->isDue($now), 'PINGREQ became due too early');
+}
+assertMqttKeepalive($keepalive->isDue(115),
+    'continuous inbound packets postponed the client PINGREQ');
+
+$keepalive->markSent(115);
+assertMqttKeepalive(!$keepalive->isDue(129), 'PINGREQ repeated before its interval');
+assertMqttKeepalive($keepalive->isDue(130), 'next PINGREQ was not scheduled');
+
+$invalidIntervalRejected = false;
+try {
+    new NiroMqttKeepalive(100, 0);
+} catch (InvalidArgumentException) {
+    $invalidIntervalRejected = true;
+}
+assertMqttKeepalive($invalidIntervalRejected, 'invalid keepalive interval was accepted');
+
+echo "NiroMqttKeepaliveTest passed\n";