feat: implement Phase 1 - Core Page Cache and Admin Settings UI

Implement the core page caching functionality and admin settings interface
for WP Recache plugin. This includes:

- Plugin bootstrap with activation/deactivation handlers
- Admin settings page with tabbed interface (Cache, Optimization, Advanced)
- Admin bar integration for quick cache purging
- Core page cache using output buffering
- Cache file management with mobile device separation
- Cache purge on post update/comment
- Default URL exclusions (cart, checkout, account)
- Cache statistics display
- PHPUnit test suite setup
- Docker-based development environment
- PHPStan configuration for static analysis

References:
- Closes #1 (Core Page Cache)
- Closes #8 (Admin Settings UI)

tests/bootstrap.php

@@ -0,0 +1,30 @@
+<?php
+/**
+ * PHPUnit bootstrap file.
+ *
+ * @package WP_Recache
+ */
+
+$_tests_dir = getenv('WP_TESTS_DIR');
+
+if (!$_tests_dir) {
+    $_tests_dir = rtrim(sys_get_temp_dir(), '/\\') . '/wordpress-tests-lib';
+}
+
+if (!file_exists("{$_tests_dir}/includes/functions.php")) {
+    echo "Could not find {$_tests_dir}/includes/functions.php\n";
+    exit(1);
+}
+
+require_once "{$_tests_dir}/includes/functions.php";
+
+/**
+ * Load the plugin.
+ */
+function _load_plugin() {
+    require dirname(__DIR__, 2) . '/wp-recache.php';
+}
+
+tests_add_filter('muplugins_loaded', '_load_plugin');
+
+require "{$_tests_dir}/includes/bootstrap.php";