Surface / Surface and Bundle

Who owns what

	Surface	Bundle (placed)
State	Owned elsewhere; the Surface reads it	Owned by the parent Model at the Link's path
Transitions	None. A Surface has no update, no private Model, no Message wrapping	The child's `update`, compiled to Foldkit's own child fold
Messages	An allowlist of constructible union variants	Wrapped variants of the parent union, routed to the slice by wrapper
Runtime behavior	None — pure contract	None added — compiles to existing lifts, no store/reducer/runtime of its own
May read across Submodels	Yes; that is the point	No; a placement renders and folds its own slice

The shorthand from the Surface README still holds:

Optic       = structural focus
Surface     = observation + capability contract
Submodel    = state-machine ownership boundary

A Bundle is the packaging of the third line: a Submodel written once and placed through a Link.