Adds form.grid (container-query column layout), form.modal (composed
modal+form+footer), form.actions (standardized footer), and a
color-aware ghost button variant. Rewrites the Boost guidance with
form/modal/checkbox/dd decision rules calibrated from humblebear, plus
canonical /atom/docs demos. All additive and backward-compatible.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>