Decide whether containment is host CSS or a library option

[QuentinRoy]

Question

Is containment host CSS, or a library option?

Throughout charting, overflow: hidden on the parent was treated as the opt-out for hosts that want the drawing contained — it is CSS-native, it needs no API, and it already has a consumer (live-surface.tsx:354). That was asserted while explaining the change, never actually decided.

#263's configurability rule does not obviously settle it: pure look is a custom property, behaviour is an option, even when the behaviour changes what you see. Containment reads as either — it is a clipping rectangle (look) that changes what feedback the user gets from a gesture that leaves the box (behaviour).

Decide whether containment is:

• host CSS only (overflow: hidden on the parent, documented as the contract),
• a library option,
• a custom property,
• or some combination, e.g. host CSS is the mechanism but the library documents and tests it as part of its interface.

Blocked on the element question: if the wrapper turns out to be the drawing surface, containment may be a property of the wrapper's own style rather than something the host does to parent.