fix(hyperframes): gate scaffold clip visibility

[RayJiang4S]

Summary

• hide generated scaffold clips by default and explicitly show/hide each cut on the GSAP timeline
• keep text-card entrance motion while avoiding blank first-frame snapshots
• add scaffold HTML assertions for active-cut visibility timing

Test plan

• .venv/bin/python -m pytest tests/tools/test_hyperframes_compose.py -q
• Generated a text-card scaffold and captured snapshots at 1.25s, 3.75s, and 6.25s to confirm only the active cut is visible