⚡ Optimize ShareableArray.unshift and addOrSetItem

[google-labs-jules]

💡 What: Replaced manual shifting loops in ShareableArray.unshift, deleteItem, and addOrSetItem with Uint8Array.prototype.copyWithin. Refactored addOrSetItem to handle in-place replacement efficiently without unnecessary shift-left then shift-right operations.

🎯 Why: Manual loop shifting is slow in JS compared to native memmove. The original replacement logic was O(N) due to double shifting, making operations like unshift O(N*K). The optimization makes replacement O(1) (excluding allocation) and shifting O(N) native.

📊 Measured Improvement: Benchmark shows unshift performance improved from ~1.64ms to ~0.015ms (>100x speedup) for adding 10 items to a 10,000 element array.

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PR created automatically by Jules for task 2911820460188290781 started by @pverscha

[google-labs-jules]

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