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

REQUIREMENTS.md

@@ -561,7 +561,7 @@ 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. Full-page shell composition may pass already-rendered hemplate fragments through explicit `SafeHtml` fields; handlers should prefer slot/resource render helpers for effect payloads.
+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, and already-safe fragments may be joined without downgrading to `String`; 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/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 {

slhx-core/src/lib.rs

@@ -141,6 +141,20 @@ impl SafeHtml {
     pub fn as_str(&self) -> &str {
         &self.0
     }
+
+    pub fn join(fragments: impl IntoIterator<Item = SafeHtml>) -> Self {
+        fragments.into_iter().collect()
+    }
+}
+
+impl FromIterator<SafeHtml> for SafeHtml {
+    fn from_iter<T: IntoIterator<Item = SafeHtml>>(iter: T) -> Self {
+        let mut html = String::new();
+        for fragment in iter {
+            html.push_str(fragment.as_str());
+        }
+        Self(html)
+    }
 }
 
 impl AsRef<str> for SafeHtml {

slhx-core/tests/effect_batch.rs

@@ -93,6 +93,18 @@ fn slot_html_requires_explicit_safe_html() {
     assert_eq!(payload, Payload::Html(String::from("<strong>ok</strong>")));
 }
 
+#[test]
+fn safe_html_joins_only_explicit_safe_fragments() {
+    // req: html_safety/001 req: html_safety/002
+    let html = SafeHtml::join([
+        SafeHtml::trusted("<main>"),
+        SafeHtml::trusted("<strong>ok</strong>"),
+        SafeHtml::trusted("</main>"),
+    ]);
+
+    assert_eq!(html.as_str(), "<main><strong>ok</strong></main>");
+}
+
 #[test]
 fn param_names_format_generated_param_names() {
     // req: codegen/003