Replace ad-hoc icon scripts with a single source-of-truth manifest
(scripts/assets.yaml) and a Python generator that renders every app icon,
store-listing graphic, MSIX tile, favicon, and the Play feature graphic from
two SVG sources. Compositions (icon + "Hammer" wordmark in Kingthings
Trypewriter) are defined once as percentages of canvas and reused across the
Play feature graphic, MSIX wide/splash tiles, and the Snap featured banner.
Adds the missing snap store icon, fixes the snap desktop icon to the correct
256x256 hicolor size, and routes uploads-only assets to build/store-assets/
(gitignored).
See docs/ASSET-GENERATION.md for the manifest schema.
