fix: address Phase 1 code review findings

- Serve cache via template_redirect so feeds/404/REST are excluded and
  query conditionals work
- Bypass cache on query strings; key files by md5(host+path) to avoid
  collisions and support multisite
- Write wp-content/wp-recache-cache.php drop-in server and set WP_CACHE
  in wp-config.php so advanced-cache.php actually runs
- Only remove advanced-cache.php on deactivation when it is ours
- Respect DONOTCACHEPAGE, non-200 responses; bots read but never warm
  cache
- Fix comment purge (comment_post_ID) and heartbeat purge ($data arg)
- Fix exclude URLs textarea parsing; purge all when exclusions change
- Enqueue admin purge script on every admin-bar page, not just settings
- Sync tests, add TESTING.md manual test guide

tests/test-cache.php

@@ -10,6 +10,11 @@ class WP_Recache_WPCache_Test extends WP_UnitTestCase {
     public function test_cache_creates_file_on_first_request() {
         $cache = new WP_Recache_WPCache();
 
+        $_SERVER['HTTP_HOST'] = 'example.com';
+        $_SERVER['REQUEST_URI'] = '/test-page/';
+        $_SERVER['HTTPS'] = 'off';
+        unset($_SERVER['HTTP_USER_AGENT']);
+
         $url = 'https://example.com/test-page/';
         $path = wp_recache_get_cache_path($url);
 
@@ -59,6 +64,11 @@ class WP_Recache_WPCache_Test extends WP_UnitTestCase {
     public function test_cache_file_is_created_with_correct_permissions() {
         $cache = new WP_Recache_WPCache();
 
+        $_SERVER['HTTP_HOST'] = 'example.com';
+        $_SERVER['REQUEST_URI'] = '/test-page/';
+        $_SERVER['HTTPS'] = 'off';
+        unset($_SERVER['HTTP_USER_AGENT']);
+
         $url = 'https://example.com/test-page/';
         $path = wp_recache_get_cache_path($url);