Unify annotation schemas (EV bcq, VQA + Temporal field union) so HF viewer can load (#18)

VANTAGE-Bench update: prompts + question-only annotations + Vantage2DPointing rename + test split + data/README + CHANGELOG (#17)

Restructure data/: drop Spatial, rename to dense_captioning / event_verification / pointing / referring / temporal_localization / vqa (#4)