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/v0/src/main.rs

@@ -185,6 +185,7 @@ fn all_examples() -> 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),