Require the current transaction owner for store operations

[OskarEichler]

Summary

Check Mutex#owned? instead of locked? before store operations. A second thread currently reads/writes a transaction it does not own and can change its abort flag.

Reproduction and verification

Hold a transaction in thread A, then call [], fetch, []=, delete, keys, key?, commit and abort from thread B. All must raise PStore::Error, and A must still commit its original value. Both thread_safe settings are covered using bounded queue synchronization.

• External focused corpus: 18 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

Cross-thread calls outside their own transaction now raise the documented not-in-transaction error. Legitimate owner access and serialized transaction calls remain supported.