The shell becomes a window manager — peek-deck, co-existence split, and a draggable seam

augment-it's shell stops taking turns. Where it used to mount one federated frontend at a time and destroy it to show another, it now mounts several at once and tiles them like a desktop window manager. Three layout modes: a peek-deck where the focused app sits at 90% with its neighbours peeking from the edges and hover-expanding, a co-existence split where two apps share the viewport behind a draggable seam, and plain full-width. Layout state persists per-user behind a seam the shared-auth store will back later.