refactor(storage): remove dead HoleGranularityCoarse helper

[HardcoreMonk]

요약

D3 리뷰(PR #88)에서 발견한 dead code 정리: storage.HoleGranularityCoarse 헬퍼를 제거한다. behavior 변경 없음.

근거

HoleGranularityCoarse(dir) bool은 production 호출자가 0이었다 — D3 가드 두 곳(판독측 overlaySparseDiff, 창설측 snapshotsDirCoarse)은 진단 메시지에 granularity 값 g가 필요해 holeGranularityFn/ProbeHoleGranularity를 직접 호출하고 predicate(err != nil || g > HoleGranularityFine)를 inline으로 적용한다. bool 헬퍼는 그 자리에 맞지 않는다. 유일한 참조는 테스트의 중복 assertion 2건(각각 바로 위의 직접 g/err assertion이 이미 커버) — 함께 제거해도 커버리지 손실 없음.

변경

• internal/storage/hole_granularity.go: HoleGranularityCoarse 함수 제거. holeGranularityFn(production 사용 + 테스트 주입 seam, snapshot.go:437)은 유지, doc 갱신.
• internal/storage/hole_granularity_test.go: 중복 HoleGranularityCoarse assertion 2건 제거(직접 g/err assertion으로 대체 주석).

검증

go build ./... OK, go vet OK, go test ./internal/storage/ PASS, gofmt -l . empty. lifecycle: Lightweight(dead-code 제거, behavior/contract 무변경, 1-2 파일).

Summary by CodeRabbit

• Bug Fixes

  • Improved handling of storage capability checks, including cases where probing fails or storage uses fine-grained support.
  • Ensured read-side safeguards consistently apply safe behavior when storage characteristics cannot be determined.

• Refactor

  • Simplified internal storage capability evaluation without changing expected user-facing behavior.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 851e0429-f5e5-4e26-9fd4-b90a90b11e91

📥 Commits

Reviewing files that changed from the base of the PR and between 42d78c4 and 05a562c.

📒 Files selected for processing (2)

• internal/storage/hole_granularity.go
• internal/storage/hole_granularity_test.go

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📝 Walkthrough

Walkthrough

The exported HoleGranularityCoarse helper was removed and replaced by the holeGranularityFn probe seam. Tests now validate fine and probe-error cases directly through returned granularity and error semantics.

Changes

Hole granularity probing

Layer / File(s)	Summary
Probe seam and regression assertions internal/storage/hole_granularity.go, internal/storage/hole_granularity_test.go	The coarse helper was replaced with holeGranularityFn, while tests now assert fine and probe-failure behavior using probe results directly.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and accurately summarizes the main change: removing the unused HoleGranularityCoarse helper.
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.

✨ 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 chore/remove-dead-hole-coarse

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