refactor: normalize ShowAddButton to IsAddButtonVisible

Rename ShowAddButton() -> IsAddButtonVisible() to align with WinUI's
Is* naming convention for boolean properties.

Keep IsTabStop() as-is (already matches WinUI naming).
Old ShowAddButton kept as [Obsolete] shim for backward compat.

Fixes #268

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sundaramramaswamy

plugins/reactor/skills/reactor-dsl/references/reactor.api.txt

@@ -447,8 +447,8 @@ T.WithKey<T>(string key) → T
 T.WithToolTip<T>(Element tooltip) → T
 T.WithTransitions<T>(Transition[] transitions) → T
 T.XYFocusKeyboardNavigation<T>(XYFocusKeyboardNavigationMode mode) → T
+TabViewElement.IsAddButtonVisible(bool visible = true) → TabViewElement
 TabViewElement.Set(Action<TabView> configure) → TabViewElement
-TabViewElement.ShowAddButton(bool visible = true) → TabViewElement
 TeachingTipElement.Set(Action<TeachingTip> configure) → TeachingTipElement
 TemplatedFlipViewElement<T>.Set<T>(Action<FlipView> configure) → TemplatedFlipViewElement<T>
 TemplatedGridViewElement<T>.Set<T>(Action<GridView> configure) → TemplatedGridViewElement<T>

skills/reactor.api.txt

@@ -447,8 +447,8 @@ T.WithKey<T>(string key) → T
 T.WithToolTip<T>(Element tooltip) → T
 T.WithTransitions<T>(Transition[] transitions) → T
 T.XYFocusKeyboardNavigation<T>(XYFocusKeyboardNavigationMode mode) → T
+TabViewElement.IsAddButtonVisible(bool visible = true) → TabViewElement
 TabViewElement.Set(Action<TabView> configure) → TabViewElement
-TabViewElement.ShowAddButton(bool visible = true) → TabViewElement
 TeachingTipElement.Set(Action<TeachingTip> configure) → TeachingTipElement
 TemplatedFlipViewElement<T>.Set<T>(Action<FlipView> configure) → TemplatedFlipViewElement<T>
 TemplatedGridViewElement<T>.Set<T>(Action<GridView> configure) → TemplatedGridViewElement<T>

src/Reactor/Elements/ElementExtensions.cs

@@ -865,9 +865,17 @@ public static GridViewElement SelectionMode(this GridViewElement el, ListViewSel
 
     // ── TabView sugar ───────────────────────────────────────────────
 
-    public static TabViewElement ShowAddButton(this TabViewElement el, bool visible = true) =>
+    /// <summary>
+    /// Controls visibility of the "add tab" button in a TabView.
+    /// </summary>
+    public static TabViewElement IsAddButtonVisible(this TabViewElement el, bool visible = true) =>
         el with { IsAddTabButtonVisible = visible };
 
+    /// <inheritdoc cref="IsAddButtonVisible"/>
+    [Obsolete("Use IsAddButtonVisible() — imperative 'Show*' naming is deprecated (see #268).")]
+    public static TabViewElement ShowAddButton(this TabViewElement el, bool visible = true) =>
+        el.IsAddButtonVisible(visible);
+
     // ── Key ─────────────────────────────────────────────────────────
 
     public static T WithKey<T>(this T el, string key) where T : Element =>

tests/Reactor.Tests/ElementExtensionsCoverageTests.cs

@@ -378,7 +378,15 @@ public void ListView_GridView_SelectionMode()
     }
 
     [Fact]
-    public void TabView_ShowAddButton_Sugar()
+    public void TabView_IsAddButtonVisible_Sugar()
+    {
+        var el = TabView([]).IsAddButtonVisible(false);
+        Assert.False(el.IsAddTabButtonVisible);
+    }
+
+    [Fact]
+    [Obsolete("Tests the deprecated ShowAddButton shim")]
+    public void TabView_ShowAddButton_Sugar_Obsolete_Shim()
     {
         var el = TabView([]).ShowAddButton(false);
         Assert.False(el.IsAddTabButtonVisible);