docs(examples): mark page shell html boundaries

Document that remaining SafeHtml use in examples is limited to full-page shell composition of already-rendered hemplate fragments, while effect payloads use render helpers.

req: html_safety/002

req: view/001

examples/kanban/src/main.rs

@@ -225,6 +225,7 @@ fn parse_column(value: Option<&str>) -> usize {
 }
 
 fn page_html(board: &BoardState) -> String {
+    // Explicit full-page composition boundary for already-rendered hemplate fragments.
     // req: html_safety/002 req: view/001
     ui::board::lower_html(render_template(&Board {
         options: render_options(),