Named width tiers (mobile/narrow/regular/wide) with body max-width easing
between them; grid pages opt in via BaseLayout `wide`. Masonry is now
absolute-positioned with FLIP glides on column-count changes: targets are
computed from the *final* container width (reading --max, which flips
instantly while max-width tweens), mid-glide column changes retarget
instead of being dropped, and container height eases so content below
rides along.
Claude-Session: https://claude.ai/code/session_01ADR9r5rw5NuvvnCxfYbMuc