refactor: make getRelatedTo return an iterator

Convert `getRelatedTo` from returning an array to returning an
`IterableIterator`. This improves memory efficiency by yielding
entities lazily instead of allocating a new array on every call.

src/bt/runner.ts

@@ -52,8 +52,8 @@ function clearSubtree(world: World, entity: Entity): void {
   }
 }
 
-function childrenOf(world: World, parent: Entity): Entity[] {
-  return world.getRelatedTo(parent, ChildOf);
+function* childrenOf(world: World, parent: Entity): IterableIterator<Entity> {
+  yield* world.getRelatedTo(parent, ChildOf);
 }
 
 function parentOf(world: World, child: Entity): Entity | null {
@@ -230,10 +230,9 @@ export class TaskRunner {
   }
 
   private _executeRandom(entity: Entity): void {
-    const children = childrenOf(this._world, entity);
-
-    // Check if any child already reached a terminal
-    for (const child of children) {
+    // Single pass: check terminals and collect eligible children
+    const eligible: Entity[] = [];
+    for (const child of childrenOf(this._world, entity)) {
       if (isTerminal(this._world, child)) {
         const status = terminalStatus(this._world, child)!;
         this._finish(
@@ -246,13 +245,14 @@ export class TaskRunner {
         );
         return;
       }
+      if (
+        !this._world.has(child, Running) &&
+        !this._world.has(child, Scheduled)
+      ) {
+        eligible.push(child);
+      }
     }
 
-    // Pick a random child that isn't already running
-    const eligible = children.filter(
-      (c) => !this._world.has(c, Running) && !this._world.has(c, Scheduled),
-    );
-
     if (eligible.length > 0) {
       const pick = eligible[Math.floor(Math.random() * eligible.length)];
       this._world.add(pick, Scheduled);
@@ -261,7 +261,7 @@ export class TaskRunner {
   }
 
   private _executeRepeat(entity: Entity): void {
-    const children = childrenOf(this._world, entity);
+    const children = [...childrenOf(this._world, entity)];
 
     // Repeat expects exactly one child
     if (children.length === 0) return;

src/world.ts

@@ -388,17 +388,22 @@ export class World {
   /**
    * Get all source entities that point to `target` via this relationship.
    */
-  getRelatedTo(target: Entity, rel: RelationshipDef): Entity[] {
+  *getRelatedTo(
+    target: Entity,
+    rel: RelationshipDef,
+  ): IterableIterator<Entity> {
     const ti = entityIndex(target);
-    if (!this._isAlive(ti, target)) return [];
+    if (!this._isAlive(ti, target)) return;
 
     const rev = this._relReverse.get(rel._key);
-    if (!rev) return [];
+    if (!rev) return;
 
     const sources = rev.get(ti);
-    if (!sources) return [];
+    if (!sources) return;
 
-    return [...sources].map((si) => makeEntity(si, this._generations[si]));
+    for (const si of sources) {
+      yield makeEntity(si, this._generations[si]);
+    }
   }
 
   /**