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)

includes/Cache/WPCache.php

@@ -0,0 +1,135 @@
+<?php
+/**
+ * Core page cache functionality.
+ *
+ * @package WP_Recache
+ */
+
+defined('ABSPATH') || exit;
+
+class WP_Recache_WPCache {
+
+    /**
+     * Initialize cache hooks.
+     */
+    public function init() {
+        add_action('init', [$this, 'start_output_buffering']);
+        add_action('shutdown', [$this, 'end_output_buffering']);
+
+        add_action('save_post', [$this, 'purge_post_cache']);
+        add_action('wp_insert_comment', [$this, 'purge_post_cache']);
+
+        add_filter('heartbeat_received', [$this, 'purge_cache_onheartbeat']);
+    }
+
+    /**
+     * Start output buffering.
+     */
+    public function start_output_buffering() {
+        if (wp_recache_should_exclude()) {
+            return;
+        }
+
+        if (wp_recache_is_cache_enabled()) {
+            $this->serve_cached_page();
+        }
+    }
+
+    /**
+     * Serve cached page if available.
+     */
+    private function serve_cached_page() {
+        $url = $this->get_current_url();
+        $cache_path = wp_recache_get_cache_path_with_device($url);
+
+        if (file_exists($cache_path)) {
+            $modified_time = filemtime($cache_path);
+            $cache_duration = wp_recache_get_option('cache_duration', 3600);
+
+            if ((time() - $modified_time) < $cache_duration) {
+                header('X-WP-Recache: HIT');
+                header('X-WP-Recache-Time: ' . date('Y-m-d H:i:s', $modified_time));
+                readfile($cache_path);
+                exit;
+            }
+        }
+
+        header('X-WP-Recache: MISS');
+        ob_start([$this, 'cache_output']);
+    }
+
+    /**
+     * Cache the output.
+     *
+     * @param string $output Page output.
+     * @return string Original output.
+     */
+    public function cache_output($output) {
+        if (empty($output)) {
+            return $output;
+        }
+
+        $url = $this->get_current_url();
+        $cache_path = wp_recache_get_cache_path_with_device($url);
+        $cache_dir = dirname($cache_path);
+
+        if (!is_dir($cache_dir)) {
+            wp_mkdir_p($cache_dir);
+        }
+
+        file_put_contents($cache_path, $output);
+
+        return $output;
+    }
+
+    /**
+     * End output buffering.
+     */
+    public function end_output_buffering() {
+        if (ob_get_level()) {
+            ob_end_flush();
+        }
+    }
+
+    /**
+     * Purge cache for a post.
+     *
+     * @param int $post_id Post ID.
+     */
+    public function purge_post_cache($post_id) {
+        $url = get_permalink($post_id);
+
+        if ($url) {
+            wp_recache_delete_cache($url);
+        }
+
+        wp_recache_delete_cache(home_url('/'));
+    }
+
+    /**
+     * Purge cache on heartbeat.
+     *
+     * @param array $response Heartbeat response.
+     * @return array Modified response.
+     */
+    public function purge_cache_onheartbeat($response) {
+        if (isset($response['wp_recache_purge'])) {
+            wp_recache_delete_all_cache();
+        }
+
+        return $response;
+    }
+
+    /**
+     * Get current request URL.
+     *
+     * @return string Current URL.
+     */
+    private function get_current_url() {
+        $protocol = is_ssl() ? 'https' : 'http';
+        $host = $_SERVER['HTTP_HOST'];
+        $uri = $_SERVER['REQUEST_URI'];
+
+        return $protocol . '://' . $host . $uri;
+    }
+}