fix: align eager, lazy, and LCP image hints with HTML standard

[seonghobae]

What

• Removes forced decoding="async" from eager first-viewport images and the single declared LCP candidate, leaving the standards-defined missing-value default (auto) to the user agent.
• Retains decoding="async" for explicitly lazy-loaded images.
• Adds non-vacuous regression contracts for eager, lazy, and high-priority LCP-candidate image sets.
• Records the decision, limitations, measurement plan, and APA 7th references in docs/doctoring/image-rendering-hints.md.

Why

The HTML Living Standard defines decoding as a preference hint whose missing-value default is auto. It does not guarantee a main-thread or background-thread execution path. fetchpriority independently influences fetch priority. This PR therefore avoids claiming that removing decoding="async" alone universally improves LCP.

Verification required at the exact current head

• python3 -m pytest tests/
• all required security, static-analysis, and repository-policy checks
• independent non-author approval on the exact current head
• deployment performance measured separately under representative device and network conditions

References

• WHATWG. (2026, July 20). HTML living standard: The img element.
• Osmani, A., Sohoni, L., Meenan, P., & Pollard, B. (2023, November 14). Optimize resource loading with the Fetch Priority API. web.dev.

This PR supersedes the stale duplicate #100 by carrying forward its valid standards reasoning and stronger non-vacuous tests on the current base.

Summary by CodeRabbit

• 성능 개선

  • 첫 화면의 주요 이미지가 브라우저 기본 디코딩 전략을 사용하도록 조정했습니다.
  • 지연 로딩 이미지는 비동기 디코딩을 유지하고, 단일 LCP 후보에는 높은 우선순위를 적용했습니다.

• 문서

  • 이미지 렌더링 힌트와 성능 측정 기준을 문서화했습니다.

• 테스트

  • 이미지 로딩·디코딩 동작과 주요 섹션의 레이아웃 안정성 검증을 강화했습니다.

[google-labs-jules]

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[coderabbitai]

📝 Walkthrough

Walkthrough

홈페이지 이미지 렌더링 힌트 계약을 정리했습니다. eager 이미지는 기본 auto 디코딩을 사용합니다. lazy 이미지는 decoding="async"를 유지합니다. 단일 LCP 후보의 fetchpriority="high" 규칙, 관련 문서, 마크업, 정적 회귀 테스트를 함께 갱신했습니다.

Changes

이미지 렌더링 힌트

Layer / File(s)	Summary
렌더링 힌트 계약 docs/doctoring/image-rendering-hints.md, .jules/bolt.md, CHANGELOG.md	이미지 로딩, fetchpriority, decoding 계약을 문서화했습니다. 단일 LCP 후보는 fetchpriority="high"를 사용합니다. eager 이미지는 decoding을 생략합니다. lazy 이미지는 decoding="async"를 유지합니다. 정적 테스트 계약과 배포 후 측정 한계도 기록했습니다.
홈페이지 이미지 마크업 index.html	브랜드 로고와 히어로 이미지에서 decoding="async" 속성을 제거했습니다. 다른 로딩 동작과 속성은 유지됩니다.
이미지 회귀 검증 tests/test_styles.py	홈페이지 img 속성을 수집하는 헬퍼와 타입 힌트를 추가했습니다. eager 이미지의 decoding 생략, lazy 이미지의 decoding="async", 단일 LCP 후보의 eager·high-priority 조건을 각각 검증합니다. 기존 CSS 회귀 테스트는 유지하고, tall section의 contain-intrinsic-size 검증을 추가했습니다.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	PR 제목은 eager, lazy 및 LCP 이미지 힌트 정합성 개선이라는 주요 변경 사항을 간결하고 정확하게 설명합니다.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch bolt-lcp-svg-optimization-4787273986862805491

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