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

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()))
     }