feat(mix-detail): extract shared ReleaseHeroOverlay; Mix cover becomes overlaid 600px square hero (Direction B)

DeepDrftPublic.Client/Controls/ReleaseDetailScaffold.razor.cs

@@ -60,6 +60,14 @@ public partial class ReleaseDetailScaffold : ComponentBase
     /// <summary>Optional medium-specific metadata block, rendered under a divider when present.</summary>
     [Parameter] public RenderFragment? MetaContent { get; set; }
 
+    /// <summary>
+    /// Gate for the header region (masthead + play, or a custom <see cref="Header"/>). A composer that
+    /// carries title/artist/play elsewhere — e.g. Mix overlays them on its hero — sets this false to
+    /// suppress the duplicate. A gate, not a layout flag, so it is slot-consistent with
+    /// <see cref="ShowMeta"/> / <see cref="ShowShareRow"/> (Phase 9 §5.3). Defaults to shown.
+    /// </summary>
+    [Parameter] public bool ShowHeader { get; set; } = true;
+
     /// <summary>
     /// Gate for the metadata block. Lets a consumer supply a <see cref="MetaContent"/> fragment but
     /// suppress the divider + block when its data is empty (slot fragments cannot be conditionally