Preserve file modes during atomic saves

[OskarEichler]

Summary

Copy the existing store mode to the temporary file before an atomic rename. This prevents an atomic transaction from replacing a deliberately restricted file with the process-default creation mode.

Reproduction

Create a PStore, chmod it to modes such as 0600, 0640, 0750 or 06750, then commit an atomic transaction. Current master recreates the path with a mode derived from 0666 and umask; this change retains the original permission and special-mode bits.

Verification

• Existing suite: 13 tests / 26 assertions on Ruby 4.0.6 and 3.2.11.
• External metadata model: 21 mode, uid and gid checks per Ruby, including special bits.
• Exact 14-path gem package build, install and load on both Rubies.
• Syntax and existing lint baseline checked; no repository tests changed.

Compatibility and limits

The change only affects changed atomic writes. It preserves permission bits, not ownership, ACLs, extended attributes, hard-link identity, or broader crash, NFS and Windows semantics. Local bounded measurements show the extra stat and chmod cost only on atomic replacement.