refactor: establish Animation folder feature - #323

Tracking issue: #342

Stack context

Part 13 of the Option 5 folder-feature stack.

• Previous: refactor: consolidate Messaging folder feature #322
• Next: another dependency-clean DOM-adjacent folder

Review this PR as the incremental diff against #322.

Summary

• add the target src/animation folder on the unqualified internal Animation feature
• move Animation, AnimationEffect, and DocumentTimeline out of the legacy broad DOM folder
• add the supported WebAPI.Animation feature with its WebAPI.Event dependency
• preserve all three flat public module paths and binding signatures

Temporary state

• this PR introduces no compatibility aliases, file exclusions, or module slices; the Animation folder membership and public implication are final
• DOMNodes interfaces currently call Animation APIs directly; that dependency will be resolved explicitly when the DOMNodes boundary is implemented rather than hidden in this layer

Review focus

• the WebAPI.Animation -> WebAPI.Event + Animation implication
• complete Animation folder membership
• unchanged public APIs

Verification

• node_modules/.bin/rescript clean
• node_modules/.bin/rescript build --prod --features DOM,Event,Animation
• npm run build
• npm test
• npm run format:check
• git diff --check