Variants need grouping — "Onboarding A/B/C" should read as one labeled row, not a soup of loose frames. Without a section primitive every board re-implements titled lanes, inline rename, and a stable group id for agents to address.

ArtBoard.Section groups pieces into a titled, horizontally-scrolling row. It's an authoring marker (renders nothing itself — the board compiles it into the value) with an inline-editable title, optional subtitle, and a stable id exposed as data-fa-section so an agent can target the group directly.

Wrap <ArtPiece> children in <ArtBoard.Section id="hero" title="Hero variants" subtitle="A/B/C">. When the board is driven by value, sections come from ArtBoardValue.sections instead and the children are ignored.