Phase 9 Wave 4: ARCHIVE nav + Cuts/Sessions/Mixes pages + MixWaveformVisualizer

Replaces flat RELEASES/SESSIONS/MIXES nav with ARCHIVE dropdown (PageRoute.Children,
one-level cap, dual-role node). Adds /archive overview, /cuts (AlbumsView + medium
filter; /albums redirects), /sessions + /sessions/{id} (hero-dominant), /mixes +
/mixes/{id} (MixWaveformVisualizer full-page background). Extracts ReleaseDetailScaffold
from TrackDetail (invariant trio). PersistentComponentState bridge on all new pages.
Click-to-seek seam designed on MixWaveformVisualizer (inert until wired).

DeepDrftPublic.Client/Controls/ReleaseDetailScaffold.razor

@@ -0,0 +1,40 @@
+@namespace DeepDrftPublic.Client.Controls
+
+@* Invariant trio shared by every medium's detail page: a back link, a masthead (title + artist),
+   a play/share affordance row wired to the streaming player, and slots for the medium-specific
+   hero visual and metadata block. TrackDetail and the Session/Mix detail pages all compose this;
+   per-medium variance rides the Hero and MetaContent render fragments. *@
+
+<div class="deepdrft-track-detail-container">
+
+    <MudLink Href="@BackHref" Typo="Typo.body2" Class="deepdrft-track-detail-back">
+        &larr; @BackLabel
+    </MudLink>
+
+    <MudStack Row AlignItems="AlignItems.Start" Justify="Justify.SpaceBetween" Style="margin: 2rem 0 1.5rem;">
+        <div class="deepdrft-track-detail-masthead">
+            <MudText Typo="Typo.h3">@Title</MudText>
+            <MudText Typo="Typo.h6" Color="Color.Primary">@Artist</MudText>
+        </div>
+
+        @* Play + share only make sense once a playable track is resolved. *@
+        @if (Track is not null)
+        {
+            <MudStack Row AlignItems="AlignItems.Center" Spacing="1">
+                <SharePopover EntryKey="@Track.EntryKey" />
+                <PlayStateIcon Track="@Track" Size="Size.Large" Color="Color.Secondary" OnToggle="@PlayTrack" />
+            </MudStack>
+        }
+    </MudStack>
+
+    @Hero
+
+    @if (MetaContent is not null && ShowMeta)
+    {
+        <MudDivider />
+        <div class="deepdrft-track-detail-meta">
+            @MetaContent
+        </div>
+    }
+
+</div>