Do not treat truncated read-only stores as empty files

[OskarEichler]

Summary

Only treat EOFError as a new empty store when the file is actually empty. A truncated Marshal stream currently appears to be an empty store in read-only transactions.

Reproduction and verification

Checked every nonempty prefix of a small Marshal store, plus a truly empty file. Truncated prefixes must be rejected; an empty file must remain a valid empty store.

• External focused corpus: 33 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

Read-only callers now receive the underlying EOFError for truncated nonempty data instead of silently receiving an empty store. Genuine empty/missing read-only stores retain their behavior.