Add per-scene thumbnail gallery (227 previews)

Update README+Croissant for 167 HSSD scenes / all-semantics / 3.3 TB; drop 3 non-HSSD test scenes (apartment_1, skokloster-castle, van-gogh-room) for license incompatibility

Add dataset card and Croissant metadata with RAI fields for NeurIPS 2026 D&B submission