Preserve animation build order when updating effects - #163

Summary

• Update selected animation builds in place while preserving their existing order and object identity.
• Keep neighboring builds unchanged when an effect or timing setting is edited.
• Add unit and end-to-end coverage for mixed animation sequences and effect updates.

Testing

• Added unit coverage for build order, IDs, and neighboring build preservation.
• Added Playwright coverage confirming mixed animation sequences remain intact after changing an effect.