Make JSON file writes atomic

[giwaov]

Summary

• Add an atomic file-write helper that writes through a temporary file in the destination directory before renaming it into place.
• Use the helper for shared JSON persistence, config writes, and peer-book writes.
• Add focused coverage for replacement behavior and parent directory creation.

Why

Fixes #189. A crash or interrupted write could leave persisted JSON files such as the peer book truncated, causing startup failures when the file was read again.

Testing

• git diff --check

Go tests were not run locally because go is not installed on this machine.