Preserve false entries in KeyValue subtree proxies

[OskarEichler]

Summary

Use key ownership rather than truthiness when reading cached KeyValue subtree entries.

The current proxy re-reads cached false values and reports them as absent through has_key?. Explicit key checks preserve boolean translations and avoid repeated store reads.

Verification

• focused external false-value ownership/cache model
• 1,607 base-suite runs / 2,674 assertions
• 1,888 Rails-suite runs / 3,020 assertions
• all 46 shipped Ruby files pass syntax checks