修复图片改造尺寸保持

保持图片改造提交与画布回填的原图尺寸
在 VectorEngine 边界对齐非 16 倍数参考图并恢复目标尺寸
补充前后端回归测试与编辑器尺寸契约文档

src/components/image-editor/ImageCanvasGenerationSubmissionModel.test.ts

@@ -122,6 +122,8 @@ describe('ImageCanvasGenerationSubmissionModel', () => {
     const sourceLayer = createLayer({
       objectKey: 'generated-character-drafts/editor/source.png',
       assetKind: 'character',
+      originalWidth: 1537,
+      originalHeight: 1025,
     });
 
     const plan = buildImageGenerationSubmissionPlan({
@@ -150,7 +152,7 @@ describe('ImageCanvasGenerationSubmissionModel', () => {
       normalizedPrompt: '把当前图改成雨天',
       sourceLayer,
       editInput: {
-        size: '1024x768',
+        size: '1537x1025',
         model: IMAGE_MODEL_GPT_IMAGE_2,
       },
       result: {