Add animation playback speed control

[han4wluc]

Summary

• Add action-level animations.playback.speed for authoring 2x, 3x, 0.5x style playback without duplicating animation resources.
• Normalize and validate playback speed across all presentation animation refs.
• Scale effective persistent animation expiry by playback speed.
• Add visual VT coverage that checks fast and slow playback from the same animation resource.
• Bump package feature version to 1.21.0.

Dependency

• Depends on Add animation playback speed control route-graphics#293 and the route-graphics 1.22.0 release for renderer-side playback-speed support.

Testing

• bun run lint
• bun run test
• bun run build
• env RTGL_VT_DEBUG=true rtgl vt screenshot --wait-event vt:ready --item visual/playback-speed.yaml --concurrency 1 --timeout 60000
• rtgl vt report --item visual/playback-speed.yaml