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/kanban/src/lib.rs

@@ -27,7 +27,7 @@ mod tests {
     #[test]
     fn kanban_board_updates_generated_slot() {
         fn render_board() -> impl IntoEffect {
-            board::slots::board.html(super::ui::render(&empty_board()))
+            super::ui::put(board::slots::board, &empty_board())
         }
 
         let effect = inspect(render_board());