feat(bt): implement subtree status clearing in TaskRunner

Introduce `clearSubtree` to recursively reset status for an entity and
all its descendants. This ensures that when a task reaches a terminal
state, its entire branch is properly reset.

Also refactor the Tetris example to use a sequential task node within
the behavior tree.

examples/tetris/main.ts

@@ -55,6 +55,9 @@ world.add(game, TickTimer);
 // Create blessed UI
 const ui = createUI();
 
+// Spawn the first piece
+spawnPiece();
+
 // ── Command handlers ─────────────────────────────────
 const commands = new CommandQueue(world);
 
@@ -186,6 +189,7 @@ const runner = new TaskRunner(world);
 
 // Build the BT structure:
 //   root (repeat)
+//     └── seq (sequential)
 //           ├── handleInput (leaf)
 //           ├── gravityTick (leaf)
 //           └── render (leaf)
@@ -193,17 +197,21 @@ const runner = new TaskRunner(world);
 const root = world.spawn();
 world.add(root, Task, { kind: "repeat" });
 
+const seq = world.spawn();
+world.add(seq, Task, { kind: "sequential" });
+world.relate(seq, ChildOf, root);
+
 const handleInputTask = world.spawn();
 world.add(handleInputTask, Task, { kind: "leaf" });
-world.relate(handleInputTask, ChildOf, root);
+world.relate(handleInputTask, ChildOf, seq);
 
 const gravityTask = world.spawn();
 world.add(gravityTask, Task, { kind: "leaf" });
-world.relate(gravityTask, ChildOf, root);
+world.relate(gravityTask, ChildOf, seq);
 
 const renderTask = world.spawn();
 world.add(renderTask, Task, { kind: "leaf" });
-world.relate(renderTask, ChildOf, root);
+world.relate(renderTask, ChildOf, seq);
 
 // ── Leaf handlers ────────────────────────────────────
 runner.onLeaf = (_w, entity) => {

src/bt/runner.ts

@@ -44,6 +44,14 @@ function clearStatus(world: World, entity: Entity): void {
   if (world.has(entity, Running)) world.remove(entity, Running);
 }
 
+/** Recursively clear status from an entity and all its descendants. */
+function clearSubtree(world: World, entity: Entity): void {
+  clearStatus(world, entity);
+  for (const child of childrenOf(world, entity)) {
+    clearSubtree(world, child);
+  }
+}
+
 function childrenOf(world: World, parent: Entity): Entity[] {
   return world.getRelatedTo(parent, ChildOf);
 }
@@ -258,9 +266,9 @@ export class TaskRunner {
 
     const child = children[0];
 
-    // If child reached a terminal, reset it and schedule again
+    // If child reached a terminal, reset the entire subtree and schedule again
     if (isTerminal(this._world, child)) {
-      clearStatus(this._world, child);
+      clearSubtree(this._world, child);
       this._world.add(child, Scheduled);
       return;
     }