Keys carry an easing for the span to the next key: linear, step, ease in, ease out, ease in-out (#101)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/store/serialize.ts

@@ -18,7 +18,7 @@ import {
 } from '../graph';
 import { isParticleNodeDef } from '../particles/nodes';
 import { isShaderNodeDef } from '../shader/nodes';
-import { normalizeKeys } from '../model/animation';
+import { isKeyInterpolation, normalizeKeys } from '../model/animation';
 import type { AnimationKey, AnimationTrack, Asset, StateMachineAsset, TextureFrame } from '../model/Asset';
 import { isAssetType } from '../model/AssetTypes';
 import { isFontFile } from '../model/font';
@@ -473,7 +473,11 @@ function normalizeTracks(raw: unknown): AnimationTrack[] {
       const key = asRecord(k);
       const frame = Number(key.frame);
       if (!Number.isFinite(frame) || !isPrimitive(key.value)) continue;
-      keys.push({ frame, value: key.value });
+      const kept: AnimationKey = { frame, value: key.value };
+      // `linear` is what no field means, so it is not kept; an unknown one reads as linear, too.
+      if (isKeyInterpolation(key.interpolation) && key.interpolation !== 'linear')
+        kept.interpolation = key.interpolation;
+      keys.push(kept);
     }
     const track: AnimationTrack = { id: t.id, target, property: t.property, keys: normalizeKeys(keys) };
     if (t.relative === true) track.relative = true;