feat: add htmx server-side plumbing

src/Htmx/HxRedirectResponse.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Htmx;
+
+use Symfony\Component\HttpFoundation\Response;
+
+/**
+ * Response that makes htmx perform a full page navigation.
+ *
+ * htmx never follows a normal 3xx itself -- the browser does, transparently, and htmx then swaps
+ * whatever the final destination returned, which is a whole page in the wrong place. So a redirect
+ * has to be handed to htmx as an instruction instead: a 200 carrying HX-Redirect.
+ *
+ * Anything returning this from an event listener must also call
+ * ExceptionEvent::allowCustomResponseCode(), or the kernel overwrites the 200.
+ */
+class HxRedirectResponse extends Response
+{
+    public function __construct(string $url)
+    {
+        parent::__construct('', Response::HTTP_OK, ['HX-Redirect' => $url]);
+    }
+}