refactor(reatom): idiomatic withResizeObserver exposing plain Size state

[vekilibot]

Summary

• withResizeObserver now exposes plain Size state ({ width, height }) instead of the live ResizeObserverEntry platform object — Reatom state should be immutable, serializable plain data
• Renamed the derived atom sizeEntry → size (domain data, not the platform type) and simplified the entry lookup to entries.at(-1) since a single node is observed
• Kept the target() read inside memo — it's load-bearing: memo recreates the observable (and disconnects the old ResizeObserver) when the node changes
• Updated the demo AppShell (target.size()?.height ?? 0), the mirrored create-karkas template copy, and added a patch changeset

Test plan

• mise run ci (or repo equivalent) on the branch
• Manual: resize the window with the app-shell header visible, confirm appShell.headerHeight still tracks it

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