feat(bt): add declarative tree builder

Introduce `buildTree` to allow defining behaviour trees using a
declarative `TreeDef` object instead of manual entity spawning and
relationship wiring. This simplifies tree construction and manages
leaf handlers internally.

examples/tetris/main.ts

@@ -1,7 +1,7 @@
 // ── Tetris: BT-driven game loop with command-based input ──
 //
 // Architecture:
-//   Behaviour Tree (TaskRunner) — controls game flow:
+//   Behaviour Tree (buildTree) — controls game flow:
 //     root (repeat)
 //       └── seq (sequential)
 //             ├── handleInput (leaf)     — reads queued commands, mutates state
@@ -19,7 +19,7 @@
 //   npx tsx examples/tetris/main.ts
 
 import { World } from "../../src/index";
-import { TaskRunner, Task, ChildOf } from "../../src/bt/index";
+import { buildTree } from "../../src/bt/index";
 import { CommandQueue } from "../../src/commands/index";
 
 import { Board, Piece, Score, GameOver, Paused, TickTimer } from "./components";
@@ -208,64 +208,48 @@ function lockAndSpawn(): void {
 }
 
 // ── Behaviour Tree ───────────────────────────────────
-const runner = new TaskRunner(world);
-
-// Build the BT structure:
-//   root (repeat)
-//     └── 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, seq);
-
-const gravityTask = world.spawn();
-world.add(gravityTask, Task, { kind: "leaf" });
-world.relate(gravityTask, ChildOf, seq);
-
-const renderTask = world.spawn();
-world.add(renderTask, Task, { kind: "leaf" });
-world.relate(renderTask, ChildOf, seq);
-
-// ── Leaf handlers ────────────────────────────────────
-runner.onLeaf = (_w, entity) => {
-  if (entity === handleInputTask) {
-    commands.execute();
-    runner.succeed(entity);
-  } else if (entity === gravityTask) {
-    if (world.hasSingleton(GameOver) || world.hasSingleton(Paused)) {
-      runner.succeed(entity);
-      return;
-    }
-    const timer = world.getSingleton(TickTimer);
-    timer.accumulator += 16;
-    if (timer.accumulator >= timer.interval) {
-      timer.accumulator -= timer.interval;
-      if (hasActivePiece()) {
-        const piece = world.getSingleton(Piece);
-        const board = world.getSingleton(Board);
-        if (!collides(board.grid, piece.shape, piece.x, piece.y + 1)) {
-          piece.y++;
-        } else {
-          lockAndSpawn();
-        }
-      }
-    }
-    runner.succeed(entity);
-  } else if (entity === renderTask) {
-    render(world, ui);
-    runner.succeed(entity);
-  }
-};
+const runner = buildTree(world, {
+  kind: "repeat",
+  child: {
+    kind: "sequential",
+    children: [
+      {
+        kind: "leaf",
+        run: () => {
+          commands.execute();
+        },
+      },
+      {
+        kind: "leaf",
+        run: () => {
+          if (world.hasSingleton(GameOver) || world.hasSingleton(Paused)) {
+            return;
+          }
+          const timer = world.getSingleton(TickTimer);
+          timer.accumulator += 16;
+          if (timer.accumulator >= timer.interval) {
+            timer.accumulator -= timer.interval;
+            if (hasActivePiece()) {
+              const piece = world.getSingleton(Piece);
+              const board = world.getSingleton(Board);
+              if (!collides(board.grid, piece.shape, piece.x, piece.y + 1)) {
+                piece.y++;
+              } else {
+                lockAndSpawn();
+              }
+            }
+          }
+        },
+      },
+      {
+        kind: "leaf",
+        run: () => {
+          render(world, ui);
+        },
+      },
+    ],
+  },
+});
 
 // ── Input → Command mapping ──────────────────────────
 const keyToCommand: Partial<Record<Key, typeof MoveLeft>> = {
@@ -287,7 +271,7 @@ startInput(ui.screen, (key) => {
 });
 
 // ── Game loop ────────────────────────────────────────
-runner.schedule(root);
+runner.schedule((runner as any).root);
 
 const TICK_MS = 16;
 const interval = setInterval(() => {

src/bt/index.ts

@@ -12,3 +12,6 @@ export type { TaskKind } from "./task";
 
 export { TaskRunner } from "./runner";
 export type { LeafHandler, TerminalHandler } from "./runner";
+
+export { buildTree } from "./tree-def";
+export type { TreeDef, LeafResult } from "./tree-def";

src/bt/tree-def.ts

@@ -0,0 +1,91 @@
+import type { World, Entity } from "../index";
+import { Task, ChildOf } from "./task";
+import { TaskRunner } from "./runner";
+
+// ── Types ─────────────────────────────────────────────
+
+/** Return value from a leaf's `run` function. */
+export type LeafResult = "success" | "fail" | "cancel" | void;
+
+/** Declarative behaviour-tree definition. */
+export type TreeDef =
+  | { kind: "leaf"; run: (world: World) => LeafResult }
+  | { kind: "sequential"; children: TreeDef[] }
+  | { kind: "parallel"; children: TreeDef[] }
+  | { kind: "selector"; children: TreeDef[] }
+  | { kind: "random"; children: TreeDef[] }
+  | { kind: "repeat"; child: TreeDef };
+
+// ── Builder ───────────────────────────────────────────
+
+/**
+ * Recursively materialize a `TreeDef` into ECS entities and return a
+ * fully-wired `TaskRunner`.
+ *
+ * Leaf `run` functions are called each tick the leaf is active:
+ * - `"success"` → leaf succeeds
+ * - `"fail"`    → leaf fails
+ * - `"cancel"`  → leaf (and subtree) is cancelled
+ * - `undefined`  → leaf stays Running, re-invoked next tick
+ *
+ * @example
+ * ```ts
+ * const runner = buildTree(world, {
+ *   kind: "repeat",
+ *   child: {
+ *     kind: "sequential",
+ *     children: [
+ *       { kind: "leaf", run: () => { doWork(); } },
+ *       { kind: "leaf", run: () => { render(); } },
+ *     ],
+ *   },
+ * });
+ * runner.schedule(runner.root);
+ * setInterval(() => runner.tick(), 16);
+ * ```
+ */
+export function buildTree(world: World, def: TreeDef): TaskRunner {
+  const leafHandlers = new Map<Entity, (world: World) => LeafResult>();
+
+  function build(def: TreeDef, parent?: Entity): Entity {
+    const entity = world.spawn();
+
+    if (def.kind === "leaf") {
+      world.add(entity, Task, { kind: "leaf" });
+      leafHandlers.set(entity, def.run);
+    } else if (def.kind === "repeat") {
+      world.add(entity, Task, { kind: "repeat" });
+      build(def.child, entity);
+    } else {
+      world.add(entity, Task, { kind: def.kind });
+      for (const child of def.children) {
+        build(child, entity);
+      }
+    }
+
+    if (parent) {
+      world.relate(entity, ChildOf, parent);
+    }
+
+    return entity;
+  }
+
+  const root = build(def);
+
+  const runner = new TaskRunner(world);
+
+  runner.onLeaf = (_w, entity) => {
+    const handler = leafHandlers.get(entity);
+    if (!handler) return;
+    const result = handler(_w);
+    if (result === "success") runner.succeed(entity);
+    else if (result === "fail") runner.fail(entity);
+    else if (result === "cancel") runner.cancel(entity);
+    // undefined → leaf stays Running, re-invoked next tick
+  };
+
+  // Stash the root entity on the runner for convenience
+  (runner as any).root = root;
+
+  return runner;
+}