Unified window shadow

One shadow field for the whole surface, computed per pixel: shadows never stack where windows overlap, occlusion follows stacking order, and the penumbra can grow with elevation. Drag to move, drag borders to resize, click to raise, ⧉/× to duplicate/close. All parameters, backends (raw JS / WASM / WebGL), and re-render cadence are selectable; each phase is measured with the Performance API.

• vanilla.html — windows as raw DOM
• react.html — windows as React (VDOM) components, same engine
• solid.html — windows as Solid (fine-grained signals) components, same engine

See the repository for the vocabulary (CONTEXT.md) and docs/adr/ for the layering decision.