feat(core): add the Accordion component

[edwintantawi]

A stack of disclosure rows in the same token-driven shape as the rest of the set. contained groups the rows on one surface and divides them; separated gives each its own outline. exclusive opens one row at a time, and a row can be disabled.

Underneath it is <details> and <summary>, so the keyboard, the screen reader and find-in-page behaviour are the platform's rather than ours - and exclusive is nothing but the shared name attribute, so no state tracks which row is open.

Four decisions worth writing down:

• Rows arrive as an items array rather than as children. A compound <Accordion.Item> would have to read the group's variant and its exclusive group name out of React context on every render, and the package README advertises that nothing here does a context lookup in the render path. That paragraph is extended rather than quietly falsified.
• The chevron mirrors the element's own open state through onToggle instead of turning in CSS. stylex.when.ancestor('[open]') compiles to :where(.marker[open] *), which reaches every chevron under an open row and would rotate the closed ones of a nested Accordion with it.
• contained draws its dividers by painting the group in the border token and letting a gap of the same width show through between opaque rows. No :first-child rule to exempt the first one, and a theme that thickens its outlines thickens the dividers to match.
• A disabled row takes the pointer away from its summary and drops it out of the tab order rather than cancelling the click. Enter on a focused summary is a click, so both routes had to go; the not-allowed cursor sits on the row, since a summary that ignores the pointer cannot set one.

The example app gains an Accordion section showing both variants, the exclusive group and the disabled row.