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/SessionDetail.razor.css

@@ -0,0 +1,14 @@
+/* Hero-dominant: a wide 16:9 image rather than the square cover used on track detail. */
+.session-detail-hero {
+    margin: 0 auto 2rem;
+    overflow: hidden;
+}
+
+/* session-detail-hero-img rides on MudPaper (child Razor component); ::deep pierces its output. */
+::deep .session-detail-hero-img {
+    width: 100%;
+    aspect-ratio: 16 / 9;
+    background-size: cover;
+    background-position: center;
+    background-repeat: no-repeat;
+}