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/MixWaveformVisualizer.razor

@@ -0,0 +1,35 @@
+@namespace DeepDrftPublic.Client.Controls
+
+@* Full-page background waveform for a Mix release. Deliberately NOT the player-bar peak-bar idiom
+   (SpectrumVisualizer / LevelMeterFab own that): this renders a single continuous mirrored
+   silhouette filling the viewport behind the detail content. Fetches its own datum from
+   api/release/{id}/mix/waveform. The played portion is washed with the progress overlay driven by
+   PlaybackPosition; the click-to-seek seam (OnSeek + bindable PlaybackPosition) is wired here for a
+   future wave even though click handling does not ship yet. *@
+
+<div class="mix-waveform-bg @(_profile is null ? "mix-waveform-bg--empty" : null)">
+    @if (_profile is not null)
+    {
+        <svg class="mix-waveform-svg"
+             viewBox="0 0 @ViewBoxWidth 100"
+             preserveAspectRatio="none"
+             role="img"
+             aria-label="Waveform">
+            <defs>
+                <clipPath id="@_clipId">
+                    <path d="@_silhouettePath" />
+                </clipPath>
+            </defs>
+
+            @* Base silhouette. *@
+            <path d="@_silhouettePath" class="mix-waveform-fill" />
+
+            @* Played-portion wash: a full-height rect clipped to the silhouette, width tracking
+               PlaybackPosition. Clamped to [0, 1]. *@
+            <rect x="0" y="0"
+                  width="@(ClampedPosition * ViewBoxWidth)" height="100"
+                  class="mix-waveform-played"
+                  clip-path="url(#@_clipId)" />
+        </svg>
+    }
+</div>