refactor(admin): remove download config settings page and chain

- Drop the download config menu entry, route and schema from the settings SPA
- Remove the PHP schema section, the server-side alias validation and the
  reserved-alias list that only served that section
- Unregister aksrt_download_config from standalone import/export keys
- Drop the contract tests that locked the removed page; the Download.php
  runtime contracts stay untouched

src/Admin/SchemaProvider.php

@@ -301,19 +301,6 @@ class SchemaProvider {
             }
         }
 
-        if ($domain === 'permissions' && $section === 'download') {
-            $alias = trim((string) ($data['url_alias'] ?? ''));
-            if ($alias === '') {
-                $errors[] = __('下载链接 URL 别名不能为空', 'aksrt');
-            } elseif (strlen($alias) > 48) {
-                $errors[] = __('下载链接 URL 别名长度不能超过 48 个字符', 'aksrt');
-            } elseif (!preg_match('/^[A-Za-z0-9_-]+$/', $alias)) {
-                $errors[] = __('下载链接 URL 别名只能包含英文字母、数字、连字符和下划线，禁止中文、空格和 / \\ ? # & 等特殊符号', 'aksrt');
-            } elseif (in_array(strtolower($alias), $this->reserved_download_aliases(), true)) {
-                $errors[] = __('该别名与系统保留路径冲突，请更换其他别名以避免路由 404', 'aksrt');
-            }
-        }
-
         return ['valid' => empty($errors), 'errors' => $errors];
     }
 
@@ -443,18 +430,6 @@ class SchemaProvider {
 			&& empty($parts['fragment']);
 	}
 
-    /**
-     * Top-level slugs that must never be shadowed by the download route.
-     */
-    private function reserved_download_aliases(): array {
-        return [
-            'wp-json', 'wp-admin', 'wp-content', 'wp-includes',
-            'admin', 'login', 'logout', 'register', 'lostpassword',
-            'feed', 'search', 'author', 'page', 'comments',
-            'sitemap', 'sign', 'cgi-bin',
-        ];
-    }
-
     public function read_settings(string $domain, string $section = ''): array {
         if ($section !== '') {
             $schema = $this->get_section_schema($domain, $section);