feat(p12-w4): ambient visualizer slot on scaffold + popover controls on all detail hosts

Add optional Ambient slot to ReleaseDetailScaffold (full-bleed layer behind content; absent = no regression). Cut mounts it + popover; Session mounts the engine directly behind its hero; Mix swaps its inline knob-bar for the lava-lamp popover.

DeepDrftPublic.Client/Controls/ReleaseDetailScaffold.razor.cs

@@ -51,6 +51,16 @@ public partial class ReleaseDetailScaffold : ComponentBase
     /// <summary>Medium-specific hero visual (cover art, hero image, or waveform background).</summary>
     [Parameter] public RenderFragment? Hero { get; set; }
 
+    /// <summary>
+    /// Optional full-bleed ambient layer rendered BEHIND the scaffold content (Phase 12 §3c/§3f mode B).
+    /// A host that wants a living environment behind hero+content — e.g. Cut supplying a
+    /// <c>WaveformVisualizer</c> — places it here. The mounted layer positions itself fixed/inset:0
+    /// (its own CSS), so the scaffold only promotes its content into a foreground stacking context above
+    /// it. Absent = today's plain background, no regression (Liskov). Mode A (Mix) and mode C (the
+    /// NowPlaying card) mount the engine without this slot — see §3f.
+    /// </summary>
+    [Parameter] public RenderFragment? Ambient { get; set; }
+
     /// <summary>
     /// Optional body region rendered below the meta block — the Cut album's multi-track listing.
     /// Single-track media leave it null.