Add bounded concurrent URL transport

OpenAI gpt-5.6-sol via OpenCode implemented the bounded concurrent validated-IP transport and deterministic tests. Chris Huber reviewed and remains responsible for every line.

Author: chubes4

includes/class-static-site-importer-url-fetcher-native-handle.php

@@ -0,0 +1,18 @@
+<?php
+/**
+ * Mutable state for one nonblocking, validated URL fetch socket.
+ *
+ * @package StaticSiteImporter
+ */
+
+class Static_Site_Importer_URL_Fetcher_Native_Handle {
+	public mixed $socket;
+	public array $target;
+	public array $options;
+	public float $started;
+	public string $outbound;
+	public string $raw;
+	public bool $crypto;
+	public string $error;
+	public int $ip_index;
+}