⚡ Bolt: Optimize defragmentation and fix offset bug

[google-labs-jules]

💡 What:
Replaced manual byte-by-byte copy loops in ShareableMap.defragment and ShareableArray.defragment with Uint8Array.prototype.set using subarray views.
Fixed a bug in ShareableArray.defragment where DATA_OBJECT_OFFSET was incorrectly added twice to the currentDataStart pointer, causing gaps in the defragmented array.

🎯 Why:
The manual copy loop was slow (~62ms for 10MB/256B blocks vs ~6ms with set).
The offset bug in ShareableArray wasted 8 bytes of memory per item during defragmentation.

📊 Impact:

• ~10x speedup for the memory copy phase during defragmentation.
• Reduced memory usage in ShareableArray after defragmentation by eliminating unnecessary gaps.

🔬 Measurement:
Verified with a reproduction test that confirmed the offset bug fix (memory usage dropped from 872 to 840 bytes in test scenario).
Verified optimization with a standalone benchmark (not included in PR).
Verified full test suite passes.

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PR created automatically by Jules for task 5877455073732309065 started by @pverscha

[google-labs-jules]

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