避免超大缩放让网格步长计算死循环

previewGrid.ts 的 resolvePreviewGridStep 在 screenSpacing 溢出成 Infinity 时直接回基础步长：原先第二个 while 里 Infinity/2 仍是 Infinity，画布会卡死
previewWorkspaceZoom.test.tsx 补 Number.MAX_VALUE 缩放的回退用例

apps/ai-game-creator-shell/src/view/ui-editor/components/preview/previewGrid.ts

@@ -14,6 +14,11 @@ export function resolvePreviewGridStep(viewportScale: number): number {
 
   let step = PREVIEW_GRID_BASE_STEP;
   let screenSpacing = step * viewportScale;
+  // 极大缩放下 step * scale 会溢出成 Infinity，而 Infinity / 2 仍是 Infinity：
+  // 下面两个 while 会因此永不收敛，这里先退回基础步长。
+  if (!Number.isFinite(screenSpacing)) {
+    return PREVIEW_GRID_BASE_STEP;
+  }
   while (screenSpacing < PREVIEW_GRID_MIN_SCREEN_SPACING) {
     step *= 2;
     screenSpacing *= 2;

apps/ai-game-creator-shell/tests/previewWorkspaceZoom.test.tsx

@@ -128,6 +128,13 @@ describe('PreviewWorkspace quick zoom', () => {
     }
   });
 
+  it('falls back to the base step when the screen spacing overflows', () => {
+    // 溢出成 Infinity 时步长无法收敛：必须直接回基础步长，不能死循环。
+    expect(resolvePreviewGridStep(Number.MAX_VALUE)).toBe(
+      PREVIEW_GRID_BASE_STEP,
+    );
+  });
+
   it('applies the adaptive screen-space spacing to the preview background', () => {
     const rendered = render(<PreviewWorkspace canvas={createCanvas()} />);
     const preview = screen.getByRole('region', { name: 'UI 预览画布' });