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

@@ -5,7 +5,17 @@
    hero visual and metadata block. The Cut/Session/Mix detail pages all compose this;
    per-medium variance rides the Hero and MetaContent render fragments. *@
 
-<div class="deepdrft-track-detail-container">
+@* Ambient environment layer (Phase 12 §3c/§3f mode B): an optional full-bleed layer rendered BEHIND
+   the scaffold content. The visualizer mounted here positions itself fixed/inset:0 (its own CSS), so
+   this slot's only job is to render it before the content and put the content into a foreground
+   stacking context above it. Absent slot = no ambient layer and no foreground promotion → today's
+   plain background, byte-for-byte (Liskov). *@
+@if (Ambient is not null)
+{
+    @Ambient
+}
+
+<div class="deepdrft-track-detail-container @(Ambient is not null ? "deepdrft-track-detail-foreground" : null)">
 
     @* Two-end top row: back link (left) | optional action (right), on one SpaceBetween row. The action
        slot stays null for media that don't supply it (Track/Cut/Session), so SpaceBetween collapses to

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.

DeepDrftPublic.Client/Controls/ReleaseDetailScaffold.razor.css

@@ -3,3 +3,13 @@
     justify-content: center;
     margin-top: 1.5rem;
 }
+
+/* Foreground stacking context — applied only when an Ambient layer is present. Lifts the scaffold
+   content above the fixed full-bleed visualizer (z-index: 0) so hero + meta + body render over the
+   living waveform field (Phase 12 §3c — promotes the former Mix-bespoke .mix-detail-foreground into
+   the shared scaffold). Without an Ambient slot this class is absent and the container keeps its
+   default flow, so a slot-less host renders exactly as before. */
+.deepdrft-track-detail-foreground {
+    position: relative;
+    z-index: 1;
+}