fix: preserve Stack direct child flex semantics

[artiphishle]

Closes #70.
Refs ankhorage/zora#289.

Stack used wrapper Views to synthesize spacing between children. Those wrappers became the actual flex items and prevented authored child layout props such as flex, minHeight, minWidth, and alignSelf from participating directly in the Stack layout.

This PR makes Stack spacing container-owned:

• use the resolved React Native gap style on the Stack container;
• render authored children directly;
• remove the wrapper View path entirely;
• add a focused regression protecting the direct-child invariant;
• add a patch changeset.

This is the Surface-owned primitive fix required by the cross-platform viewport contract in ankhorage/zora#289. No compatibility or fallback path is added.