test: cover wal.rs's canonical_bytes_of_line and non-NotFound error arms

[t0k0sh1]

Summary

• canonical_bytes_of_line's None return had zero coverage at any of its three call sites (shippable_records, parse_log's interior loop, tail_record_is_complete) — confirmed by grep, no test in the repository ever exercised it. Adds one test per call site, each forcing a distinct way the byte-exact ,"crc": marker search can fail to find a field that genuinely parsed.
• reset/replay/replay_readonly previously had only their NotFound-tolerant arm tested; adds the non-NotFound propagation arm for all three (injected for reset, a real directory-in-place-of-file OS error for the two fs::read-based ones, since that call is intentionally outside the fault-injection choke point).
• Strengthens the two existing heal-failure tests (a_truncate_heal_failure_is_fatal_not_a_shrug, a_newline_heal_failure_is_fatal_not_a_shrug) to also assert the outer "could not heal ..." wrapper message, not just the injected cause underneath it.
• Adds a characterization test pinning that shippable_records does NOT dedupe a repeated seq, unlike parse_log's later-wins resolution — intentional per its own doc comment ("shipping is a byte transport, not the trust boundary"), not a gap.
• The mutation gate found one missed mutant in parse_log's tail branch (segments.len() + 1 → * 1, its own line-number computation, distinct from the interior loop's index + 1): the new tail-position marker test asserted the error message body but not the reported line number. Fixed by pinning it there too (commit a2773d9).

Part of #587 (untested-branches audit from #533). Refs #587.

Test plan

• cargo fmt
• cargo clippy --bin taguru --tests (clean)
• cargo test (full suite green, 1544 passed)
• Pre-PR mutation gate (cargo mutants --in-diff): 0 mutants (test-only diff)
• cargo mutants --file src/wal.rs: 93 tested, 65 caught, 28 unviable, 0 missed (fix verified across two independent runs — unviable count matched both times, ruling out snapshot corruption)

https://claude.ai/code/session_013NPb5cDU8ojZAKGJaq8rYy

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 4ba0960 and d4c0ef2.

📒 Files selected for processing (1)

• src/wal.rs

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📝 Walkthrough

Walkthrough

WALのCRC検証、重複sequence処理、末尾修復、およびファイルI/Oエラー伝播を検証するテストを追加しました。

Changes

WAL検証とエラー処理

Layer / File(s)	Summary
レコード検証テスト src/wal.rs	shippable_records が不正なCRC形式を拒否し、重複sequenceを保持することを検証します。CRCマーカーを復元できない内部レコードも InvalidData として拒否することを検証します。
末尾修復とI/Oエラー伝播テスト src/wal.rs	CRCマーカーや改行の修復失敗時に外側のエラーを返すことを検証します。reset、replay、replay_readonly が非NotFoundエラーを伝播することを検証します。

Estimated code review effort: 2 (Simple) | ~15 minutes

Possibly related issues

• 永続化層の重複コード・doc陳腐化 #588 — WALのCRC検証とreplay／truncate_to周辺の改善に関するIssueです。
• 永続化/WAL 層の未検証分岐 #587 — src/wal.rsのCRC、sequence処理、およびreset／replayのエラー伝播テストに直接関連します。

Possibly related PRs

• t0k0sh1/taguru#589 — src/wal.rsのWAL解析、末尾破損、切り詰め処理のテストに直接関連します。
• t0k0sh1/taguru#523 — WALの破損復旧とファイルサイズ処理のテストに関連します。

🚥 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	タイトルは、wal.rs の未カバー分岐と非 NotFound エラー処理のテスト追加という主な変更を明確に示しています。

✨ 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 587-wal-untested-branches

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