test: stable filter ordering, base TestCase reset, priority-aware remove_all_filters

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/Cf7stgTestCase.php

@@ -0,0 +1,15 @@
+<?php
+declare(strict_types=1);
+
+namespace Cf7stg\Tests;
+
+use PHPUnit\Framework\TestCase;
+
+abstract class Cf7stgTestCase extends TestCase
+{
+    protected function setUp(): void
+    {
+        parent::setUp();
+        \Cf7stgWpShimState::reset();
+    }
+}