Motion upgrades timelines with springs, callbacks

This is a substantial upgrade to Motion’s timeline model, shifting it from convenient sequencing toward a more programmable animation runtime.

• –Hardware acceleration can keep supported animations smooth while the JavaScript thread is busy.
• –Spring segments bring physics-based motion into coordinated multi-step timelines.
• –Nested, addable, movable, and removable sequences make dynamic timeline composition far more practical.
• –Direction-aware and progress callbacks enable reversible state changes, scrubbing, and synchronized application logic.
• –Relative labels and play-to-label controls reduce the timeline bookkeeping that often makes complex animation code brittle.