add v7.5.8 release notes (#23757)

[ti-chi-bot]

This is an automated cherry-pick of #23757

What is changed, added or deleted? (Required)

• Add releases/release-7.5.8.md for the TiDB 7.5.8 patch release notes.
• Add the TiDB 7.5.8 entry to TOC-tidb-releases.md, releases/_index.md, and releases/release-timeline.md.
• Fill in the TiDB 7.5.8 release date.
• This release-notes update is prepared on release-8.5 because the release-notes directory is maintained there and will be cherry-picked to master.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): add v7.5.8 release notes docs-cn#21930 ; generated from doc_4afdcdb19e49_release-7.5.8-updated-by-ai.md

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

Summary by CodeRabbit

• New Releases

  • Added TiDB v7.5.8 to the release notes index and timeline, dated September 17, 2026.

• Improvements

  • PD cluster status metrics now include per-store labels for clearer monitoring.

• Bug Fixes

  • Resolved query cleanup crashes, planning-related CPU spikes, and statistics-loading panics.
  • Fixed TiKV ingestion, CDC connection, I/O, and import stability issues.
  • Corrected PD goroutine growth and stale store metrics.
  • Fixed TiFlash data inconsistencies, BR safepoint cleanup, and TiCDC Kafka client leaks.

[ti-chi-bot]

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Advanced

Run ID: a8a42b95-b5d2-457c-b2bb-5330d6d3d486

📥 Commits

Reviewing files that changed from the base of the PR and between 54b30f0 and 893cc73.

📒 Files selected for processing (4)

• TOC-tidb-releases.md
• releases/_index.md
• releases/release-7.5.8.md
• releases/release-timeline.md

Included review availability: Your plan provides up to 4 included reviews per hour; 3 remain after this review.

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📝 Walkthrough

Walkthrough

This change adds the TiDB 7.5.8 release notes dated September 17, 2026. It documents one PD improvement and bug fixes across TiDB, TiKV, PD, TiFlash, BR, and TiCDC. It also updates three release indexes.

Changes

TiDB 7.5.8 release

Layer / File(s)	Summary
Release note content releases/release-7.5.8.md	Adds release metadata, the PD metric improvement, and bug-fix entries for TiDB, TiKV, PD, TiFlash, BR, and TiCDC.
Release index publication TOC-tidb-releases.md, releases/_index.md, releases/release-timeline.md	Adds links and timeline data for TiDB 7.5.8.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Other

Suggested reviewers: qiancai

Merge Risk: ⚪ Minimal · up to 893cc

The release notes and publication indexes are consistent and present no identified merge-blocking risk.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: adding the TiDB v7.5.8 release notes.
Description check	✅ Passed	The description covers the release-note files, affected TiDB version, release date, cherry-pick context, reference link, and AI involvement. It satisfies the required sections.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
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 unit tests (beta)

• Create PR with unit tests

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