Add optional Ambient slot to ReleaseDetailScaffold (full-bleed layer behind content; absent = no regression). Cut mounts it + popover; Session mounts the engine directly behind its hero; Mix swaps its inline knob-bar for the lava-lamp popover.
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).