feat(style): compose generated class states

Add CssClass/CssClasses with/with_if helpers so nested partial view data can compose generated CSS constants without ad hoc class strings.

req: style/003

req: component/003

examples/techdemo/src/main.rs

@@ -504,14 +504,7 @@ fn board_view(demo: &DemoState) -> BoardLanes {
 
 fn issue_card(item: &WorkItem, selected: bool) -> IssueCard {
     IssueCard {
-        class: if selected {
-            CssClasses::from([
-                classes::work_card,
-                classes::is_selected,
-            ])
-        } else {
-            CssClasses::from(classes::work_card)
-        },
+        class: classes::work_card.with_if(selected, classes::is_selected),
         id: item.id,
         title: item.title.clone(),
         stage: item.stage.label(),