✨ (base): Implementa Navbar con funcionalidad base

Añade `Navbar::with_position()` y `navbar::Position` al núcleo, con las
clases de Basic para barras fijas y pegajosas, y `navbar::Item::form()`
para mostrar un formulario dentro de la barra.

Las barras sin botón de despliegue (`Simple` y `SimpleBrandLeft`) ya no
colapsan: el punto de corte no les afecta y su menú se mantiene en
línea, con salto de línea si no cabe. En Bootsier usan `navbar-expand` y
`SimpleBrandLeft` agrupa los contenidos junto a la marca.

tests/component_navbar.rs

@@ -14,6 +14,23 @@ async fn is_not_rendered_when_empty() {
     assert!(html.is_empty());
 }
 
+// **< navbar::Item::form >*************************************************************************
+
+#[pagetop::test]
+async fn item_form_renders_the_form() {
+    let mut navbar = Navbar::simple().with_item(navbar::Item::form(
+        Form::new()
+            .with_id("search")
+            .with_action("/search")
+            .with_child(Button::submit(Lc::n("Go"))),
+    ));
+    let html = navbar.render(&mut Context::default()).await.into_string();
+
+    assert!(html.contains(r#"<form"#));
+    assert!(html.contains(r#"id="search""#));
+    assert!(html.contains("Go"));
+}
+
 // **< Navbar + FlexItem::push_end >****************************************************************
 
 #[pagetop::test]
@@ -27,6 +44,36 @@ async fn push_end_adds_an_automatic_start_margin() {
     assert!(assets.contains("_flex-item-offset_auto_{margin-inline-start:auto}"));
 }
 
+// **< Navbar::with_position >**********************************************************************
+
+#[pagetop::test]
+async fn position_is_static_by_default_and_adds_no_class() {
+    assert_eq!(Navbar::simple().position(), navbar::Position::Static);
+
+    let mut navbar = Navbar::simple().with_item(navbar::Item::nav(one_link_nav()));
+    let html = navbar.render(&mut Context::default()).await.into_string();
+
+    assert!(!html.contains("fixed-"));
+    assert!(!html.contains("sticky-"));
+}
+
+#[pagetop::test]
+async fn position_adds_its_class() {
+    for (position, class) in [
+        (navbar::Position::FixedTop, "fixed-top"),
+        (navbar::Position::FixedBottom, "fixed-bottom"),
+        (navbar::Position::StickyTop, "sticky-top"),
+        (navbar::Position::StickyBottom, "sticky-bottom"),
+    ] {
+        let mut navbar = Navbar::simple()
+            .with_position(position)
+            .with_item(navbar::Item::nav(one_link_nav()));
+        let html = navbar.render(&mut Context::default()).await.into_string();
+
+        assert!(html.contains(class), "missing `{class}` in {html}");
+    }
+}
+
 // **< Navbar::with_expand >************************************************************************
 
 #[pagetop::test]