fix: preserve running kernel cache during archive updates

tests/KernelCacheTest.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace App\Tests;
+
+use App\Kernel;
+use PHPUnit\Framework\TestCase;
+
+final class KernelCacheTest extends TestCase
+{
+    public function testAnUpdateKeepsTheRunningCacheAndSelectsANewOneForTheNextRequest(): void
+    {
+        $root = sys_get_temp_dir().'/kant-cache-'.bin2hex(random_bytes(6));
+        mkdir($root);
+        $kernel = static fn (): Kernel => new class($root) extends Kernel {
+            public function __construct(private string $root)
+            {
+                parent::__construct('prod', false);
+            }
+
+            public function getProjectDir(): string
+            {
+                return $this->root;
+            }
+        };
+        try {
+            file_put_contents($root.'/VERSION', "1.10.1\n");
+            $running = $kernel();
+            $old = $running->getCacheDir();
+            self::assertSame($old, $kernel()->getCacheDir());
+            file_put_contents($root.'/VERSION', "1.10.2\n");
+            self::assertSame($old, $running->getCacheDir(), 'The active request retains its cache even after VERSION changes.');
+            self::assertNotSame($old, $kernel()->getCacheDir(), 'A new kernel must not reuse the previous compiled container or Twig templates.');
+        } finally {
+            unlink($root.'/VERSION');
+            rmdir($root);
+        }
+    }
+}