fix: isolate cached backend values

[j6k4m8]

Summary

• snapshot iterators before caching so repeated reads remain consumable
• deep-copy cached values on storage and return to isolate nested mutation
• invalidate cached reads only after successful backend writes
• document cache synchronization and wrapped-backend concurrency scope

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks

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_{Comparing fix/cache-safe-values-82 (7746e0b) with master (c33b0b9)}