Fix Map#compute deadlock under thread contention

[kanutocd]

Summary

Fix Ratomic::Map#compute deadlocking when multiple Ruby threads contend for the same key.

When an entry is locked by another thread, compute now retries while releasing Ruby's GVL. This allows the thread executing the existing compute block to resume and release the entry lock.

Testing

• Added a deterministic regression test for concurrent Map#compute calls on the same key
• Verified the original issue reproduction
• Ran 100 repeated contention scenarios
• Full suite: 117 runs, 2,322 assertions, 0 failures, 2 skips
• cargo check passes

Fixes #27