Normalize path-like objects before retaining the store filename

[OskarEichler]

Summary

Normalize the retained filename with File.path. PStore accepts path-like inputs, but a File object is interpolated as its object representation during atomic saving, and path does not return the documented string.

Reproduction and verification

String, Pathname and open File inputs were checked for string path return and ultra_safe write/read round trips. The File input fails its atomic save before this correction.

• External focused corpus: 6 verified on this isolated branch under rbenv Ruby 4.0.6.
• Existing upstream RBENV_VERSION=4.0.6 rbenv exec rake test: 13 tests / 26 assertions, zero failures before and after.
• All files and injected failures are confined to scratch storage; no production operations.
• No repository tests added or modified under the contributor's task constraint. Other Ruby versions and native Windows were not executed locally.

Breaking-change notes and limitations

Path-like inputs are normalized to strings. Existing String inputs retain their value and behavior; no path freezing or global path-policy change.