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

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 {