feat(core): join safe html fragments

Let already-safe fragments compose without downgrading to String, and use it in the v0 page assembly example to keep rendered and lowered fragments typed at the boundary.

req: html_safety/001

req: html_safety/002

req: component/003

examples/v0/src/main.rs

@@ -182,17 +182,14 @@ fn registry(state: Arc<ExampleState>) -> HandlerRegistry {
 fn all_examples() -> SafeHtml {
     // Static `.heml` fragments are lowered by generated code before they join rendered views.
     // req: html_safety/001 req: html_safety/002 req: component/003
-    SafeHtml::trusted(
-        [
-            counter::lower(include_str!("../templates/counter.heml")),
-            todos::lower(include_str!("../templates/todos.heml")),
-            wizard::lower(include_str!("../templates/wizard.heml")),
-            auth::lower(include_str!("../templates/auth.heml")),
-            render_page_swap("Welcome", "Welcome").into_string(),
-            notifications::lower(include_str!("../templates/notifications.heml")),
-        ]
-        .join("\n"),
-    )
+    SafeHtml::join([
+        SafeHtml::trusted(counter::lower(include_str!("../templates/counter.heml"))),
+        SafeHtml::trusted(todos::lower(include_str!("../templates/todos.heml"))),
+        SafeHtml::trusted(wizard::lower(include_str!("../templates/wizard.heml"))),
+        SafeHtml::trusted(auth::lower(include_str!("../templates/auth.heml"))),
+        render_page_swap("Welcome", "Welcome"),
+        SafeHtml::trusted(notifications::lower(include_str!("../templates/notifications.heml"))),
+    ])
 }
 
 fn shell(body: SafeHtml) -> SafeHtml {