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);
 
@@ -186,24 +189,29 @@ const runner = new TaskRunner(world);
 
 // Build the BT structure:
 //   root (repeat)
-//     ├── handleInput (leaf)
-//     ├── gravityTick (leaf)
-//     └── render (leaf)
+//     └── seq (sequential)
+//           ├── handleInput (leaf)
+//           ├── gravityTick (leaf)
+//           └── render (leaf)
 
 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) => {