refactor(icons): converge every icon render on aksrt_icon_markup

aksrt_get_comment_icon(), aksrt_renderer_svg_icon() and the widget
renderer's private icon_markup() all delegate to the unified helper,
and the duplicated 24px empty-state SVGs in user-center/user-home now
come from one aksrt_empty_state_svgs(). Contract tests follow the
delegation (sprite <use> markup asserted in the helper).

inc/helpers/posts-renderer/post-data.php

@@ -241,7 +241,7 @@ if (!function_exists('aksrt_renderer_meta_number')) {
 
 if (!function_exists('aksrt_renderer_svg_icon')) {
 	function aksrt_renderer_svg_icon(string $name): string {
-		return '<svg class="icon" aria-hidden="true"><use xlink:href="#icon-' . esc_attr($name) . '"></use></svg>';
+		return aksrt_icon_markup('zibsvg-' . $name);
 	}
 }