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.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;
+}