feat(mix-visualizer): gate knob controls with Blazor @if in TopContent band; drop CSS collapse, glass, and TopRowCenter slot

DeepDrftPublic.Client/Pages/MixDetail.razor

@@ -48,18 +48,20 @@ else
                                BackLabel="All mixes"
                                ShowMeta="@(hasGenre || hasDate)"
                                ShowShareRow="false">
-            <TopRowCenter>
-                @* In-flow seven-knob control container, between the back link and the lava-lamp toggle.
-                   It grows in place (width/opacity transition) when expanded and collapses to zero
-                   footprint when closed — never a popover, drawer, or floating overlay (§7b). The
-                   container mutates the shared MixVisualizerControlState; the backdrop bridge pushes the
-                   dials. A knob drag does not collapse it — the toggle flips only on the lamp's click. *@
-                <MixVisualizerControls Expanded="@_controlsExpanded" />
-            </TopRowCenter>
+            <TopContent>
+                @* The seven-knob band lives in its own full-width area below the back/lamp top row.
+                   Blazor — not CSS — controls its visibility: it is rendered only while the lava-lamp is
+                   on, so when off it is not in the DOM at all. No background, no animation, no reflow of
+                   the row above. The band mutates the shared MixVisualizerControlState; the backdrop
+                   bridge pushes the dials. A knob drag does not toggle it — the lamp's click does. *@
+                @if (_controlsExpanded)
+                {
+                    <MixVisualizerControls />
+                }
+            </TopContent>
             <TopRightAction>
-                @* Lava-lamp button top-right, across from the back link. Toggles the in-flow control
-                   container in the center zone. The icon swaps to its FILLED variant while the
-                   container is expanded (§7f / Part B). *@
+                @* 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"
@@ -116,9 +118,9 @@ else
 @code {
     protected override string PersistKey => "mix-detail";
 
-    // Lava-lamp inline knob-bar expanded state. Pure presentation over MixVisualizerControlState — the
-    // bar discloses the seven knobs and animates open/closed; toggling it touches no control value or
-    // bridge push. The lava-lamp button's filled/outline glyph is driven off this same flag.
+    // Lava-lamp knob-band visibility. Pure presentation over MixVisualizerControlState — gates whether
+    // the seven-knob MixVisualizerControls 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;