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/Pages/MixDetail.razor

@@ -55,27 +55,12 @@ else
                                ShowHeader="false"
                                ShowMeta="false"
                                ShowShareRow="false">
-            <TopContent>
-                @* The eight-knob band lives in its own full-width area below the back/lamp top row.
-                   Phase 10 §4: the control is ALWAYS rendered; the lava-lamp toggle feeds its Visible
-                   parameter, and the control itself @if-gates the knobs while holding the container's
-                   reserved height — so content below never pops on toggle. The band mutates the shared
-                   WaveformVisualizerControlState; the backdrop bridge pushes the dials. A knob drag does not
-                   toggle it — the lamp's click does. *@
-                <WaveformVisualizerControls Visible="@_controlsExpanded" />
-            </TopContent>
             <TopRightAction>
-                @* Lava-lamp button top-right, across from the back link. Toggles the knob band below the
-                   row. The icon swaps to its FILLED variant while the band is shown (§7f / Part B). *@
-                <MudTooltip Text="Visualizer settings">
-                    <MudIconButton Icon="@(_controlsExpanded ? DDIcons.LavaLampFilled : DDIcons.LavaLamp)"
-                                   Size="Size.Large"
-                                   Color="Color.Secondary"
-                                   Disabled="@(!RendererInfo.IsInteractive)"
-                                   OnClick="@ToggleSettings"
-                                   aria-label="Visualizer settings"
-                                   aria-expanded="@_controlsExpanded" />
-                </MudTooltip>
+                @* Lava-lamp icon → popover panel, top-right across from the back link (Phase 12
+                   §3d-revised). Replaces the former inline TopContent knob-bar: the icon IS the toggle
+                   and the popover IS the panel. Mix takes the cleanest anchor case (§8e) — the popover's
+                   default bottom-right anchor opens down over the full-bleed field. *@
+                <WaveformVisualizerControlPopover />
             </TopRightAction>
             <Hero>
                 @* Cover-as-background hero with all metadata overlaid, square `mix-hero` sizing. The
@@ -128,11 +113,4 @@ else
             await PlayerService.SelectTrackStreaming(track);
         }
     }
-
-    // Lava-lamp knob-band visibility. Pure presentation over WaveformVisualizerControlState — gates whether
-    // the seven-knob WaveformVisualizerControls is rendered into the TopContent band; toggling it touches no
-    // control value or bridge push. The lava-lamp button's filled/outline glyph is driven off this flag.
-    private bool _controlsExpanded;
-
-    private void ToggleSettings() => _controlsExpanded = !_controlsExpanded;
 }