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