## Summary

Implements `__ElementAnimate` PAPI for the web platform layer, enabling
Web Animations API support in the REPL and web environment. Previously,
this function was undefined causing `ReferenceError` when worklet code
called animations.

## Changes

- **web-constants**: Added `ElementAnimatePAPI` type to
`MainThreadGlobalThis`
- **web-mainthread-apis**: Implemented `__ElementAnimate` with animation
map tracking and timing option mapping (Lynx → Web Animations API)
- **testing-environment**: Added `__ElementAnimate` implementation with
state tracking for unit tests
- **Tests**: 5 new test cases covering all animation operations
(START/PLAY/PAUSE/CANCEL/FINISH)
- **REPL example**: Interactive demo showing element animation with
pause/play/cancel/restart controls

## Verification

✅ Rebased onto main with upstream REPL site  
✅ Added comprehensive unit tests (testing-environment)  
✅ Created manual UI test in REPL at http://localhost:3003/  
✅ All pre-commit hooks passed (eslint, biome, dprint)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Element animation support: start, pause, play, cancel, finish for
named element animations; public APIs exposed for driving these
operations.
* New interactive "Element Animate" sample: pulsing element, playback
status label, control buttons, a render entry point and a simple
worklet-style router for demo controls.

* **Tests**
* Unit and end-to-end tests covering the full animation lifecycle and
state transitions.

* **Documentation**
  * Changeset entry documenting the animation PAPI addition.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->