Fix activity center collapse anchoring

[zzj3720]

Summary

• keep the activity center banner top-anchored while its body collapses so the title does not drop
• preserve the reserved activity-center height while collapsed and avoid exposing the bottom safe-area padding after collapse
• use explicit exit distance so collapsed banners dismiss below the viewport
• let the anchor shrink again when open content becomes shorter
• add focused layout helper coverage for collapse anchoring, shrink, and exit behavior

Validation

• node .yarn/releases/yarn-4.9.1.cjs typecheck
• node .yarn/releases/yarn-4.9.1.cjs test
• node .yarn/releases/yarn-4.9.1.cjs lint
• node .yarn/releases/yarn-4.9.1.cjs build:embedded (passes with existing asset size warnings)
• manual geometry smoke: expanded/collapsed top both 660px in a 900px viewport; collapsed exit bottom reaches 1016px; open content shrink moves top from 660px to 780px

[cuesurfbot]

LGTM. 上一轮提的 exit 位移和 anchor only-grow 都修了：exit 显式用 anchorHeight + safeArea + exitOffset 推出屏幕，anchor 在 open 时直接同步 measured、close 时才保留 prev。helper 测试也补到了。CI 全绿。