feat: media preview as image content and download as embedded resource

lit/media-preview downscales an image attachment via wp_get_image_editor
and returns it as ImageContent; lit/media-download returns a file (full
or a named size) as an embedded resource, rejecting files above the
upload limit. Both are direct tools (ruling Q6: embedded resources, like
ImageContent, only become content blocks for tools called directly).
Adds MediaFiles::isWithinUploads() so neither tool ever reads a file
outside wp_upload_dir()'s basedir.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

src/Integrations/Media/MediaFiles.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\Media;
+
+/**
+ * Shared file-path safety check for lit-media-preview and lit-media-download
+ * (controller ruling: never read a file outside the uploads directory, even
+ * one get_attached_file() or an intermediate size's stored path points at).
+ */
+final class MediaFiles
+{
+    public static function isWithinUploads(string $file): bool
+    {
+        $real = realpath($file);
+        if ($real === false) {
+            return false;
+        }
+
+        $uploads = wp_upload_dir();
+        $basedir = is_array($uploads) ? realpath((string) ($uploads['basedir'] ?? '')) : false;
+
+        return is_string($basedir) && str_starts_with($real, $basedir . DIRECTORY_SEPARATOR);
+    }
+}