feat: 이미지 최적화 2·3차(imageAutoSizes/imageSizesPreset) 및 문서/플랜 반영 (TDD)

- ExportOptions.imageAutoSizes 추가 및 기본 sizes 자동 적용
- ExportOptions.imageSizesPreset 추가(mobile-first/desktop-fixed)
- 우선순위: imageSizes > preset > auto > none
- 테스트 케이스 확장(image.responsive.options.test.ts)
- README/MAIN_PLAN 업데이트

README.md

@@ -63,6 +63,13 @@ The Builder can export a production-ready ZIP that contains:
 - Image Options
   - Loading: `lazy` (default) or `eager` for `<img loading>`
   - Decoding: `async` (default) or `sync` for `<img decoding>`
+  - Fetch Priority: optional `high|low` for `<img fetchpriority>`
+  - Srcset/Sizes: optional responsive sources via `<img srcset>` and `<img sizes>`
+  - Auto Sizes: when enabled and only `srcset` is provided, a sensible default is applied to `<img sizes>`: `(max-width: 640px) 100vw, 600px`
+  - Sizes Preset: choose a preset for `<img sizes>` when `imageSizes` is not set.
+    - `mobile-first`: `(max-width: 640px) 100vw, 800px`
+    - `desktop-fixed`: `1200px`
+  - Precedence: explicit `imageSizes` > `imageSizesPreset` > `imageAutoSizes` > none
 
 ### site.webmanifest and robots.txt