✨ (button): Añade Button::anchor() para navegación

- Renderiza un `<a href>` real con el aspecto de un botón, distinto del
  estilo puramente visual `button::Style::Link`.
- Renombra `ButtonKind`/`ButtonStyle` a `Kind`/`Style`.
- Añade `button::Size` (`button-sm`/`button-lg`).

tests/component_button.rs

@@ -48,17 +48,16 @@ async fn title_attribute_can_be_cleared_with_lc_none() {
 
 #[pagetop::test]
 async fn style_class_reflects_intent_and_style() {
-    let mut button =
-        Button::submit(Lc::n("Save")).with_style(button::ButtonStyle::Solid(Intent::Danger));
+    let mut button = Button::submit(Lc::n("Save")).with_style(button::Style::Solid(Intent::Severe));
     let html = button.render(&mut Context::default()).await.into_string();
 
-    assert!(html.contains("button-danger"));
+    assert!(html.contains("button-severe"));
 }
 
 #[pagetop::test]
 async fn outline_style_generates_outline_class() {
     let mut button =
-        Button::submit(Lc::n("Save")).with_style(button::ButtonStyle::Outline(Intent::Primary));
+        Button::submit(Lc::n("Save")).with_style(button::Style::Outline(Intent::Primary));
     let html = button.render(&mut Context::default()).await.into_string();
 
     assert!(html.contains("button-outline-primary"));
@@ -66,12 +65,84 @@ async fn outline_style_generates_outline_class() {
 
 #[pagetop::test]
 async fn link_style_generates_link_class_without_intent() {
-    let mut button = Button::plain(Lc::n("Cancel")).with_style(button::ButtonStyle::Link);
+    let mut button = Button::plain(Lc::n("Cancel")).with_style(button::Style::Link);
     let html = button.render(&mut Context::default()).await.into_string();
 
     assert!(html.contains("button-link"));
 }
 
+#[pagetop::test]
+async fn size_small_generates_button_sm_class() {
+    let mut button = Button::submit(Lc::n("Save")).with_size(button::Size::Small);
+    let html = button.render(&mut Context::default()).await.into_string();
+
+    assert!(html.contains("button-sm"));
+}
+
+#[pagetop::test]
+async fn size_large_generates_button_lg_class() {
+    let mut button = Button::submit(Lc::n("Save")).with_size(button::Size::Large);
+    let html = button.render(&mut Context::default()).await.into_string();
+
+    assert!(html.contains("button-lg"));
+}
+
+#[pagetop::test]
+async fn size_none_omits_size_class_by_default() {
+    let mut button = Button::submit(Lc::n("Save"));
+    let html = button.render(&mut Context::default()).await.into_string();
+
+    assert!(!html.contains("button-sm"));
+    assert!(!html.contains("button-lg"));
+}
+
+#[pagetop::test]
+async fn anchor_renders_as_a_tag_with_href() {
+    let mut button = Button::anchor(Lc::n("Edit"), "/items/1/edit");
+    let html = button.render(&mut Context::default()).await.into_string();
+
+    assert!(html.starts_with("<a "));
+    assert!(html.contains(r#"href="/items/1/edit""#));
+    assert!(html.contains("Edit"));
+}
+
+#[pagetop::test]
+async fn button_without_href_renders_as_button_tag() {
+    let mut button = Button::submit(Lc::n("Save"));
+    let html = button.render(&mut Context::default()).await.into_string();
+
+    assert!(html.starts_with("<button"));
+}
+
+#[pagetop::test]
+async fn anchor_falls_back_to_button_when_href_is_reset() {
+    // `Route::default()` resuelve a una ruta vacía; `prepare()` debe ignorar `href` y volver a
+    // renderizar un `<button>`, tal y como documenta `Button::with_href()`.
+    let mut button = Button::anchor(Lc::n("Edit"), "/items/1/edit").with_href(Route::default());
+    let html = button.render(&mut Context::default()).await.into_string();
+
+    assert!(html.starts_with("<button"));
+}
+
+#[pagetop::test]
+async fn enabled_anchor_has_no_aria_disabled_or_tabindex() {
+    let mut button = Button::anchor(Lc::n("Edit"), "/items/1/edit");
+    let html = button.render(&mut Context::default()).await.into_string();
+
+    assert!(!html.contains("aria-disabled"));
+    assert!(!html.contains("tabindex"));
+}
+
+#[pagetop::test]
+async fn disabled_anchor_omits_href_and_sets_aria_disabled() {
+    let mut button = Button::anchor(Lc::n("Edit"), "/items/1/edit").with_disabled(true);
+    let html = button.render(&mut Context::default()).await.into_string();
+
+    assert!(!html.contains("href="));
+    assert!(html.contains(r#"aria-disabled="true""#));
+    assert!(html.contains(r#"tabindex="-1""#));
+}
+
 // **< ButtonSet >**********************************************************************************
 
 #[pagetop::test]