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/SessionDetail.razor

@@ -40,11 +40,26 @@ else
     // Hero image precedence: the session's dedicated hero, then the release cover, then a placeholder.
     var heroImage = !string.IsNullOrEmpty(heroKey) ? heroKey : release.ImagePath;
 
-    <MudContainer MaxWidth="MaxWidth.Large" Class="session-detail-page">
+    @* Ambient living waveform behind the hero overlay (Phase 12 §3e option b / §3f mode B). Session does
+       NOT compose ReleaseDetailScaffold, so it mounts the shared engine directly with its own thin
+       full-bleed wrapper — the engine is single-source either way, only the mount differs (§3b). The
+       visualizer positions itself fixed/inset:0; the session-detail-foreground class lifts the content
+       above it. The bridge follows the live playing track; TrackEntryKey is the at-rest datum. *@
+    <WaveformVisualizer ReleaseEntryKey="@release.EntryKey"
+                        TrackId="@ViewModel.Track?.Id"
+                        TrackEntryKey="@ViewModel.Track?.EntryKey" />
 
-        <MudLink Href="/sessions" Typo="Typo.body2" Class="deepdrft-track-detail-back">
-            &larr; All sessions
-        </MudLink>
+    <MudContainer MaxWidth="MaxWidth.Large" Class="session-detail-page session-detail-foreground">
+
+        <div class="session-detail-top-row">
+            <MudLink Href="/sessions" Typo="Typo.body2" Class="deepdrft-track-detail-back">
+                &larr; All sessions
+            </MudLink>
+
+            @* Lava-lamp icon → popover panel (full parity, §3e/§3d-revised). Anchored top-right, clear of
+               the hero overlay and the share/play affordances overlaid on the hero below. *@
+            <WaveformVisualizerControlPopover />
+        </div>
 
         @* The overlay shows the cover thumbnail only when it differs from the resolved hero image —
            when there is no dedicated hero, heroImage already falls back to release.ImagePath, so the