feat(build): add lower-aware put helper

Generate ui::put(slot, view) for lower-aware slot HTML updates and move examples/docs/contracts to that ergonomic path instead of hand-composing slot.html(render(...)).

req: dx/006

req: component/003

req: runtime/001

req: examples/003

examples/v0/src/lib.rs

@@ -121,9 +121,9 @@ mod tests {
     fn page_swap_updates_content_and_history() {
         fn load_docs() -> impl IntoEffect {
             (
-                page_swap::slots::content.html(page_swap::render(&DocsContent {
+                page_swap::put(page_swap::slots::content, &DocsContent {
                     message: "This page was swapped.",
-                })),
+                }),
                 page_swap::slots::title.text("Docs"),
                 push("/docs"),
             )