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

REQUIREMENTS.md

@@ -558,7 +558,7 @@ what a valid business email is.
 001	Raw HTML insertion requires an explicit safe HTML type (`SafeHtml` or equivalent). Plain `String` renders as escaped text unless explicitly wrapped.
 
 ### req: html/002
-002	Hemplate-rendered output may be converted to `SafeHtml` by trusted render APIs. User input is never `SafeHtml` by default.
+002	Hemplate-rendered output may be converted to `SafeHtml` by trusted render APIs. User input is never `SafeHtml` by default. Full-page shell composition may pass already-rendered hemplate fragments through explicit `SafeHtml` fields; handlers should prefer slot/resource render helpers for effect payloads.
 
 ### req: html/003
 003	Slot render commands distinguish text payloads from HTML payloads at the type level.

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(),

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) })
 }

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),