Visual layout and placement of bins on grids

[KofTwentyTwo]

The rich logged-in end-state: a visual editor to spatially place bins onto a grid, persisted per ContainerInstance (the layout reuse axis). Per docs/PRODUCT-VISION.md, when this ships is an open decision — explicitly not the first slice.

Key tasks

• Drag/place bins on a grid in 3D; persist the arrangement per ContainerInstance; restore on load.
• Persist layout as data (never meshes), consistent with Preserve V1 generator as first Workbench item type #5.

See docs/ROADMAP.md → Phase D2.