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/Layout/DeepDrftMenu.razor.css

@@ -82,6 +82,57 @@
     color: var(--deepdrft-white);
 }
 
+/* Dual-role parent node: anchor for the parent route, positioning context for the dropdown. */
+.dd-nav-item-parent {
+    position: relative;
+}
+
+/* Hover-revealed child dropdown. Pure CSS — hidden by default, shown when the parent item is
+   hovered or contains keyboard focus. Sits directly beneath the parent link, frosted to match
+   the bar. */
+.dd-nav-dropdown {
+    position: absolute;
+    top: 100%;
+    left: 50%;
+    transform: translateX(-50%) translateY(0.25rem);
+
+    display: flex;
+    flex-direction: column;
+    align-items: stretch;
+    gap: 0.75rem;
+
+    list-style: none;
+    margin: 0;
+    padding: 1rem 1.5rem;
+    min-width: 9rem;
+
+    background: rgba(250, 250, 248, 0.96);
+    border: 1px solid var(--deepdrft-border);
+    -webkit-backdrop-filter: blur(18px);
+    backdrop-filter: blur(18px);
+
+    opacity: 0;
+    visibility: hidden;
+    pointer-events: none;
+    transition: opacity 0.18s ease, visibility 0.18s ease;
+    z-index: 101;
+}
+
+.dd-nav-dark .dd-nav-dropdown {
+    background: rgba(13, 13, 18, 0.95);
+}
+
+.dd-nav-item-parent:hover .dd-nav-dropdown,
+.dd-nav-item-parent:focus-within .dd-nav-dropdown {
+    opacity: 1;
+    visibility: visible;
+    pointer-events: auto;
+}
+
+.dd-nav-dropdown-link {
+    white-space: nowrap;
+}
+
 /* Right-side cluster */
 .dd-nav-actions {
     display: flex;
@@ -207,6 +258,11 @@
     padding: 0.6rem 0;
 }
 
+/* Indented child rows under their parent link in the mobile drawer. */
+.dd-nav-mobile-child {
+    padding-left: 1.25rem;
+}
+
 .dd-nav-links-mobile ::deep .dd-nav-cta {
     margin-top: 0.5rem;
     text-align: center;