Build the Shell Tiling & Peek-Deck — Window the Federated Frontends

The augment-it shell stops being a one-at-a-time tab switcher and becomes a desktop-window-tiling surface. Two layout modes: a peek-deck where the focused frontend sits at 90% width with its sequence neighbours peeking from the edges and hover-expanding, and a co-existence split where two frontends share the viewport at a configurable ratio with a draggable seam — entered when a prompt is run against a single record. Full-width single-frontend stays available as a third option. Layout state is User-scoped preference, persisted to localStorage now against a declared shape that the forthcoming shared-auth user store will back later.