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

lib/exporter/image.responsive.options.test.ts

@@ -35,4 +35,53 @@ describe('image responsive/export options', () => {
     expect(html).toMatch(/<img[^>]*srcset="https:\/\/example.com\/hero.jpg 1x, https:\/\/example.com\/hero@2x.jpg 2x"/)
     expect(html).toMatch(/<img[^>]*sizes="\(max-width: 640px\) 100vw, 600px"/)
   })
+
+  it('applies default sizes when srcset is provided and imageAutoSizes is true', () => {
+    const options: ExportOptions = {
+      imageSrcset: 'https://example.com/hero.jpg 1x, https://example.com/hero@2x.jpg 2x',
+      imageAutoSizes: true,
+    }
+    const { html } = exportPage(basePage, options)
+    expect(html).toMatch(/<img[^>]*srcset="https:\/\/example.com\/hero.jpg 1x, https:\/\/example.com\/hero@2x.jpg 2x"/)
+    expect(html).toMatch(/<img[^>]*sizes="\(max-width: 640px\) 100vw, 600px"/)
+  })
+
+  it('does not include sizes when srcset is provided but imageAutoSizes is false and sizes not provided', () => {
+    const options: ExportOptions = {
+      imageSrcset: 'https://example.com/hero.jpg 1x, https://example.com/hero@2x.jpg 2x',
+      imageAutoSizes: false,
+    }
+    const { html } = exportPage(basePage, options)
+    expect(html).toMatch(/<img[^>]*srcset="https:\/\/example.com\/hero.jpg 1x, https:\/\/example.com\/hero@2x.jpg 2x"/)
+    expect(html).not.toMatch(/<img[^>]*sizes="/)
+  })
+
+  it('applies preset sizes when imageSizesPreset is set (mobile-first)', () => {
+    const options: ExportOptions = {
+      imageSrcset: 'https://example.com/hero.jpg 1x, https://example.com/hero@2x.jpg 2x',
+      imageSizesPreset: 'mobile-first',
+    }
+    const { html } = exportPage(basePage, options)
+    expect(html).toMatch(/<img[^>]*sizes="\(max-width: 640px\) 100vw, 800px"/)
+  })
+
+  it('applies desktop-fixed preset when selected', () => {
+    const options: ExportOptions = {
+      imageSrcset: 'https://example.com/hero.jpg 1x, https://example.com/hero@2x.jpg 2x',
+      imageSizesPreset: 'desktop-fixed',
+    }
+    const { html } = exportPage(basePage, options)
+    expect(html).toMatch(/<img[^>]*sizes="1200px"/)
+  })
+
+  it('explicit imageSizes overrides presets and auto', () => {
+    const options: ExportOptions = {
+      imageSrcset: 'https://example.com/hero.jpg 1x, https://example.com/hero@2x.jpg 2x',
+      imageAutoSizes: true,
+      imageSizesPreset: 'desktop-fixed',
+      imageSizes: '(max-width: 768px) 100vw, 700px',
+    }
+    const { html } = exportPage(basePage, options)
+    expect(html).toMatch(/<img[^>]*sizes="\(max-width: 768px\) 100vw, 700px"/)
+  })
 })