Compare the missing-default sentinel by identity

[OskarEichler]

Summary

Compare the missing-default sentinel by identity, avoiding arbitrary equality methods on a caller-supplied default object.

Reproduction and verification

A default whose == always returns true is currently mistaken for PStore::Error and raises. Checked that object, the actual sentinel, nil, false, zero and string defaults.

• 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

Caller defaults are returned as supplied unless they are the actual PStore::Error sentinel. No API removal.