cloud: add tidb-x-cloud.202603.1.md

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Add tidb-x-cloud.202603.1.md

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Summary by CodeRabbit

• New Features
  • Added partial index support and LATERAL syntax parsing.
  • Added table aliases for FOR UPDATE OF.
  • Added MySQL 8.4 Dumpling compatibility.
  • Added table affinity and shared-lock foreign-key checks.
• Performance & Stability
  • Optimized lossy DDL operations and index lookup pushdown.
  • Improved background-task resource controls.
• Observability
  • Added fine-grained slow-query logging.
• Documentation
  • Published release notes for TiDB-X-CLOUD 202603.1, including compatibility and general improvements.

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• tidb-cloud/releases/tidb-x-cloud.202603.1.md

📝 Walkthrough

Walkthrough

Added the TiDB-X-CLOUD.202603.1 release entry. The notes cover release metadata, performance, stability, observability, SQL, MySQL compatibility, parsing, query processing, optimizer behavior, and operational improvements.

Changes

TiDB-X-CLOUD.202603.1 release documentation

Layer / File(s)	Summary
Release metadata tidb-cloud/releases/tidb-x-cloud.202603.1.md	Added release date, applicable plan, kernel version, preview availability, and version-numbering details.
Performance and operational features tidb-cloud/releases/tidb-x-cloud.202603.1.md	Documented lossy DDL optimization, index lookup pushdown, data affinity, foreign-key shared locks, background-task resource limits, and slow-query logging.
SQL and compatibility notes tidb-cloud/releases/tidb-x-cloud.202603.1.md	Documented table aliases in FOR UPDATE OF, partial indexes, MySQL 8.4 Dumpling support, and LATERAL syntax parsing.
General improvements tidb-cloud/releases/tidb-x-cloud.202603.1.md	Documented improvements to parsing, analysis, query processing, optimizer behavior, partitioned tables, PD behavior, configuration, metrics, and HTTP responses.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Suggested labels: contribution, translation/no-need, area/tidb-cloud, for-cloud-release

Suggested reviewers: lilin90

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the addition of the TiDB X Cloud release notes file.
Description check	✅ Passed	The description follows the template and explains the file addition, but no applicable TiDB version checkbox is selected.
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.

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• tidb-cloud/releases/tidb-x-cloud.202603.1.md