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/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