shm: sbo head 池翻倍、data 区 ×64 升根、删 value 时 PUNCH_HOLE 回收（#15） - #22

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/