feat(bt): support generator leaves and delta time

Introduce support for generator functions in leaf tasks, allowing
them to yield for multi-frame execution. The `TaskRunner` now accepts
a delta time (`dt`) parameter, which is passed through to leaf
handlers.

Additionally, a `Cancel` symbol is introduced to allow leaf tasks to
explicitly cancel their subtree via a thrown error.

examples/tetris/main.ts

@@ -221,21 +221,24 @@ const runner = buildTree(world, {
       },
       {
         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();
+        *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 (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();
+                }
               }
             }
           }
@@ -275,7 +278,7 @@ runner.schedule((runner as any).root);
 
 const TICK_MS = 16;
 const interval = setInterval(() => {
-  runner.tick();
+  runner.tick(TICK_MS);
 }, TICK_MS);
 
 // Cleanup on exit