Add spatial UI

Build a usable surface: information and controls that remain legible, targetable, and observable in their intended pose.

1. Write the surface contract

Record every displayed value and action, its state owner, spatial anchor, appearance states, update cadence, desktop input, XR input, and observable result.

Complete this step when every value and control has a user purpose, source, anchor, and testable result.

2. Select the correct UI root

Read ../../docs/docs/manual/UI.mdx before implementation.