Animation rows can target behavior variables and shader parameters (#104)

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

src/runtime/systems/AnimationSystem.ts

@@ -1,15 +1,16 @@
 import { isHandleEnabled } from '../../core/graph';
 import {
   type AnimationPlayer,
+  animatableDefsOf,
   animatedKind,
   animationPlayheadAt,
-  isPositionKey,
+  isPositionRow,
   playbackOf,
   trackWriteAt,
 } from '../../core/model/animation';
 import type { AnimationAsset, AnimationTrack } from '../../core/model/Asset';
 import { instanceChildId, splitInstanceId } from '../../core/model/instance';
-import { findAssetOf } from '../../core/model/Project';
+import { findAsset, findAssetOf } from '../../core/model/Project';
 import type { PropertyValue } from '../../core/model/Properties';
 import type { Entity } from '../Entity';
 import type { World } from '../World';
@@ -113,7 +114,7 @@ function bind(world: World, asset: AnimationAsset, owner: Entity): Binding[] {
       entity = world.sceneId === root.id ? world.findByObjectId(track.target) : undefined;
     else if (track.target === '') entity = owner;
     else entity = world.findByObjectId(childId(owner.objectId, track.target));
-    const kind = entity && animatedKind(entity.type, key);
+    const kind = entity && kindOn(world, entity, key);
     if (entity && kind) {
       const aimed = entity;
       bindings.push({
@@ -128,6 +129,15 @@ function bind(world: World, asset: AnimationAsset, owner: Entity): Binding[] {
   return bindings;
 }
 
+/**
+ * The kind `key` has on the entity as it is now: its bag names the shader it wears, and swapping that
+ * shader at runtime takes the parameter rows with it (#104).
+ */
+function kindOn(world: World, entity: Entity, key: string): string | undefined {
+  const asset = findAsset(world.project, entity.assetId);
+  return asset && animatableDefsOf(world.project, asset, entity.props).find((d) => d.key === key)?.kind;
+}
+
 /**
  * The id a template object gets below a placed container – nested one deeper when the container is
  * content itself. `path` may run through containers placed inside the template (`a/b`).
@@ -138,12 +148,12 @@ function childId(ownerId: string, path: string): string {
 }
 
 function read(entity: Entity, key: string): PropertyValue {
-  if (isPositionKey(key)) return entity[key];
+  if (isPositionRow(entity.type, key)) return entity[key];
   return entity.props[key] ?? 0;
 }
 
 function write(entity: Entity, key: string, value: PropertyValue): void {
-  if (isPositionKey(key)) {
+  if (isPositionRow(entity.type, key)) {
     const n = Number(value);
     if (Number.isFinite(n)) entity[key] = n;
   } else entity.props[key] = value;