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/techdemo/src/main.rs

@@ -422,6 +422,8 @@ fn parse_lane(value: Option<&str>) -> usize {
 }
 
 fn page_html(demo: &DemoState) -> String {
+    // Explicit full-page composition boundary for already-rendered hemplate fragments.
+    // req: html_safety/002 req: view/001
     render_control_center(ControlCenter {
         hero: render_hero(demo),
         board: render_board(demo),
@@ -431,6 +433,7 @@ fn page_html(demo: &DemoState) -> String {
 }
 
 fn shell(body: String) -> String {
+    // Explicit full-page shell composition boundary for already-rendered hemplate fragments.
     // req: html_safety/002 req: view/001
     render_template(&AppShell { body: SafeHtml::trusted(body) })
 }