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/CutDetail.razor

@@ -43,6 +43,21 @@ else
                            BackHref="/cuts"
                            BackLabel="All cuts"
                            ShowShareRow="false">
+        <Ambient>
+            @* Ambient living waveform behind the album hero + track list (Phase 12 §3c/§3f mode B).
+               Cut is multi-track: anchor to the release's EntryKey and default to the first track by
+               TrackNumber. The bridge follows the live playing track within the release automatically
+               (keys on TrackId match OR shared ReleaseEntryKey), so the field re-renders to whichever
+               track the listener starts; TrackEntryKey is the at-rest datum before playback. *@
+            <WaveformVisualizer ReleaseEntryKey="@release.EntryKey"
+                                TrackId="@firstTrack?.Id"
+                                TrackEntryKey="@firstTrack?.EntryKey" />
+        </Ambient>
+        <TopRightAction>
+            @* Lava-lamp icon → popover panel (full parity, §3d-revised). Sits top-right across from the
+               back link, clear of the header's own Play/Share affordances below. *@
+            <WaveformVisualizerControlPopover />
+        </TopRightAction>
         <Header>
             @* Header split: meta + Play/Share on the LEFT, bordered cover on the RIGHT (spec §3.1). *@
             <div class="cut-detail-header">