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/Pages/ArchiveView.razor

@@ -0,0 +1,31 @@
+@page "/archive"
+
+<PageTitle>DeepDrft Archive</PageTitle>
+
+<div>
+    <MudContainer MaxWidth="MaxWidth.Large" Class="archive-view-container">
+        <div class="archive-grid">
+            @foreach (var medium in _media)
+            {
+                <a href="@medium.Route" class="archive-card-link">
+                    <div class="archive-card">
+                        <MudIcon Icon="@medium.Icon" Class="archive-card-icon" />
+                        <MudText Typo="Typo.h5" Class="archive-card-title">@medium.Title</MudText>
+                        <MudText Typo="Typo.body2" Class="archive-card-blurb">@medium.Blurb</MudText>
+                    </div>
+                </a>
+            }
+        </div>
+    </MudContainer>
+</div>
+
+@code {
+    private record MediumCard(string Title, string Blurb, string Route, string Icon);
+
+    private static readonly MediumCard[] _media =
+    [
+        new("Cuts", "Studio recordings — singles, EPs, and albums.", "/cuts", Icons.Material.Filled.Album),
+        new("Sessions", "Single live takes, each with its own hero image.", "/sessions", Icons.Material.Filled.Piano),
+        new("Mixes", "Long-form continuous mixes with high-resolution waveforms.", "/mixes", Icons.Material.Filled.GraphicEq),
+    ];
+}