UiTree

A retained, platform-neutral UI tree.

Every operation is confined to the thread that creates this tree. Operational methods and close are not reentrant. The state property may be read on the owner thread during an active callback. An empty tree measures to IntSize.Zero. An empty tree performs no layout work and returns empty paint, input, and semantics results. A complete description is validated before mutation. Structural validation and local validation-hook failures leave an active tree unchanged and available for retry. Any failure after validation begins reconciliation, lifecycle work, or a pipeline operation poisons the tree. The tree clears retained ownership and attempts cleanup. Cleanup preserves the primary Throwable instance and suppresses later distinct failures on it.