Deduplicate layout geometry across components and snippet exporters

[syoooo]

Box sizes and element placement formulas for each geometry (grid, line bars, loop bars, line dots, loop dots) are written three times: in the component JSX, in cssSnippet, and in svgSnippet. Prop resolution and color sampling are already shared (resolveXxxProps, xxxColors) — layout is the remaining duplication.

Proposal: extract pure layoutXxx(resolved) → { box, items[] } helpers in src/core/ consumed by all three, so a new geometry parameter is one change instead of three.