Animation: duck_anim core runtime library (workstream A)

[Clancey]

Summary

• add validated clip schema, I/O, resampling, playback, layered mixing, and hardware safety limiting
• document the .duckanim.json format and composition semantics
• cover clip validation, player envelopes, seam interpolation, mixing, safety, and resampling

Test

• animation/.venv/bin/python -m pytest animation/tests -q