test(v0): exercise checked form handler path

Use #[slhx::form] in the v0 example tests and add a checked form-handler smoke test, with a typed() helper for generated form ids so examples do not rebuild ids by hand.

req: form/001

req: form/004

req: form/006

req: derive_handler/003

examples/v0/src/lib.rs

@@ -15,6 +15,7 @@ mod tests {
     }
 
     #[derive(Clone, Debug)]
+    #[slhx::form("new_todo")]
     struct TodoInput {
         title: String,
     }
@@ -55,6 +56,20 @@ mod tests {
         assert!(matches!(effect.ops(), [Effect::Put { .. }]));
     }
 
+    // req: examples/001 req: form/001 req: form/004 req: form/006 req: derive_handler/003
+    #[test]
+    fn form_handler_is_checked_against_hemplate_form() {
+        #[slhx::handler]
+        fn add_todo(_form: slhx::Form<TodoInput>) -> impl IntoEffect {
+            ui::slots::todo_list.text("queued")
+        }
+
+        let effect = inspect(add_todo(ui::forms::new_todo.typed::<TodoInput>()));
+
+        assert!(effect.has_slot(ui::slots::todo_list));
+        assert!(matches!(effect.ops(), [Effect::Put { payload: Payload::Text(_), .. }]));
+    }
+
     // req: examples/001 req: progressive_disclosure/001 req: build/001 req: build/005 req: codegen/002
     #[test]
     fn todos_append_keyed_rows_from_form_input() {

slhx-core/src/lib.rs

@@ -881,6 +881,13 @@ impl<T> Form<T> {
         self.id
     }
 
+    pub const fn typed<U>(self) -> Form<U> {
+        Form {
+            id: self.id,
+            _marker: PhantomData,
+        }
+    }
+
     pub fn field(self, name: impl Into<String>) -> ResourceRef {
         ResourceRef::scoped(self.id, ScopeKey::Field(name.into()))
     }