Tree (WIP)

Tree owns single selection and expansion of a complete authored hierarchy. nodeKey is unique, parentKey identifies its parent, and invalid cycles or missing parents fail closed. Arrow keys, Home/End and first-letter navigation move focus; Left/Right collapse, expand or navigate to parent/child. Controlled value and expandedKeys are request-only.

This is a WIP source checkpoint. Full Adapter/Compiler, native GPUI, package consumer, accessibility and commit-bound visual acceptance remain pending.