feat(bt)!: Remove legacy tree definition format

The `LegacyTreeDef` type and its normalization path have been removed.
All tree definitions must now use the factory functions (`leaf`,
`sequential`, `parallel`, `selector`, `random`, `repeat`). The
`TreeDef` type is now simply `TaskEntityDef`.

examples/tetris/main.ts

@@ -20,7 +20,13 @@
 //   npx tsx examples/tetris/main.ts
 
 import { World } from "../../src/index";
-import { buildTree } from "../../src/bt/index";
+import {
+  buildTree,
+  leaf,
+  parallel,
+  repeat,
+  sequential,
+} from "../../src/bt/index";
 import { CommandQueue } from "../../src/commands/index";
 
 import {
@@ -64,56 +70,43 @@ registerCommands(world, commands, pieces);
 pieces.spawnPiece();
 
 // ── Behaviour Tree ───────────────────────────────────
-const runner = buildTree(world, {
-  kind: "parallel",
-  children: [
-    {
-      kind: "leaf",
-      *run() {
-        while (true) {
-          const dt: number = yield;
-          if (world.hasSingleton(GameOver) || world.hasSingleton(Paused)) {
-            continue;
-          }
-          const timer = world.getSingleton(TickTimer);
-          timer.accumulator += dt;
-          if (timer.accumulator >= timer.interval) {
-            timer.accumulator -= timer.interval;
-            if (world.hasSingleton(Piece)) {
-              const piece = world.getSingleton(Piece);
-              const board = world.getSingleton(Board);
-              if (!collides(board.grid, piece.shape, piece.x, piece.y + 1)) {
-                piece.y++;
-              } else {
-                pieces.lockAndSpawn();
-              }
+const runner = buildTree(
+  world,
+  parallel([
+    leaf(function* gravityTick() {
+      while (true) {
+        const dt: number = yield;
+        if (world.hasSingleton(GameOver) || world.hasSingleton(Paused)) {
+          continue;
+        }
+        const timer = world.getSingleton(TickTimer);
+        timer.accumulator += dt;
+        if (timer.accumulator >= timer.interval) {
+          timer.accumulator -= timer.interval;
+          if (world.hasSingleton(Piece)) {
+            const piece = world.getSingleton(Piece);
+            const board = world.getSingleton(Board);
+            if (!collides(board.grid, piece.shape, piece.x, piece.y + 1)) {
+              piece.y++;
+            } else {
+              pieces.lockAndSpawn();
             }
           }
         }
-      },
-    },
-    {
-      kind: "repeat",
-      child: {
-        kind: "sequential",
-        children: [
-          {
-            kind: "leaf",
-            run: () => {
-              commands.execute();
-            },
-          },
-          {
-            kind: "leaf",
-            run: () => {
-              render(world, ui);
-            },
-          },
-        ],
-      },
-    },
-  ],
-});
+      }
+    }),
+    repeat(
+      sequential([
+        leaf(() => {
+          commands.execute();
+        }),
+        leaf(() => {
+          render(world, ui);
+        }),
+      ]),
+    ),
+  ]),
+);
 
 // ── Input → Command mapping ──────────────────────────
 const keyToCommand: Partial<Record<Key, typeof MoveLeft>> = {