feat(broadcast-studio): Wirecast-inspired scenes and program switcher

Summary

Adds src/broadcast-studio/ — scenes, overlays, preview→program cut. No LIVE RTMP. Amplify/README + icons.

DOCUMENTED vs LIVE

See src/broadcast-studio/README.md — honest comparison; no fabricated LIVE access.

Test plan

• cd src/broadcast-studio && npm ci && npm test && npm run build (4 tests)
• Smoke the UI