@fourjs/animation

Animation system — the animation pillar (Part III). Part of fourJS.

Implements §14–§18 (§100, Part III) of docs/SPECIFICATION.md; shipped in Phase 4 (§107). All durations are in seconds; quaternion tracks interpolate by shortest-arc slerp.

What's here

• Easing — the 34-name registry (EASINGS, EASING_NAMES, resolveEasing) covering the §15 families: quadratic through quintic, sine, circular, exponential, back, elastic, bounce, and a closed-form damped spring.
• Tweens — Tween / tween() builder with repeat semantics and the TweenState lifecycle.
• Value adapters — numberAdapter, vector2Adapter/vector3Adapter/vector4Adapter, quaternionAdapter (slerp), colorAdapter, booleanAdapter/discreteAdapter, and detectAdapter; primitives return values, reference types mutate out in place (ValueAdapter.mutatesInPlace).
• Property binding — createBinding / PropertyBinding: paths resolved once, in-place writes that preserve identity and change hooks. Conflicting writers resolve last-started-wins via a claim registry shared by tweens and the mixer.
• Timelines — Timeline with elapsed-space markers, seek suppression / replayOnSeek, and nestable TimelineChild entries.
• Clips and tracks — AnimationTrack / AnimationClip (§17 shape; step, linear, and cubic interpolation) with clip events.
• Playback — AnimationMixer (prepare() + play(), translation-only root-motion option) and AnimationSystem (priority 300, fixed-step advance, auto-untracks finished animations).

Staged / not yet implemented

• State machines, blend trees, skeletal and morph-target animation (staged per plan P4-3).
• Rotational root motion (translation-only ships; staged 2026-08-02).
• Physics-animation blending itself lives in @fourjs/physics (§19), driven by @fourjs/scene's PoseTarget.

Unit tests are colocated in tests/ per §92.

Workspace name @fourjs/animation; publishes as @danielsimonjr/fourjs-animation.