fix(positions): use deterministic temp file to prevent dentry slab growth

[AruneshDwivedi]

Summary

Replaces renameio.WriteFile with a custom atomicWriteFile that uses a deterministic temp path (.tmp) instead of a unique random name on every call. This prevents unbounded kernel dentry slab growth under cgroup v2 when positions are written every 10 seconds.

Changes

• : Replaced renameio.WriteFile with atomicWriteFile using deterministic temp path
• : Added TestWritePositionFile_ReusesDentry test

Fixes #6938

[github-actions]

Signed commits report

1 of 1 commit between main and fix/positions-dentry-slab could not be fully verified:

Commit	Author	Reason	Message
9990b312	Arunesh Dwivedi	unsigned	fix(positions): use deterministic temp file to prevent dentry slab growth

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[kalleep]

Hey I believe we can use https://github.com/natefinch/atomic instead of having our own implementation. We already have this as a dependency in alloy