perf: 多 shard 锁数组按 64B 对齐，修复 >8 线程吞吐断崖（10–20 线程 1.6–3.7×） - #15

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/