3D Spatial Idea Program — Prototype 0.3.8b

0.3.8b test: clean branch from LOCKED 0.3.7o. BUILD / VIEW orbit test. preserves confirmed 0.3.7n and adds screen-space snap competition for ceilings. Wall Top/Bottom and Ceiling Edge remain legal targets; finger proximity chooses intent, with a small tie margin protecting Ceiling → Ceiling continuation in crowded structures.