Fix memory leak in SplitArray.Remove

[google-labs-jules]

💡 What:

• Updated SplitArray.Remove in spacepartition/spacemap.go to explicitly set unused elements in the backing array to nil before reslicing.
• Added a regression test TestSplitArrayRemoveLeak in spacepartition/leak_test.go to verify the fix.
• Added a benchmark BenchmarkSplitArrayRemove to monitor performance.

🎯 Why:

• When removing elements from a slice by shifting, the elements at the end of the backing array (which are no longer part of the slice) still hold references to the objects. This prevents the Garbage Collector from reclaiming them, leading to a memory leak.
• This change ensures those references are cleared, allowing memory to be freed.

📊 Measured Improvement:

• Baseline: The reproduction test confirmed that references were retained (leak present).
• After Fix: The regression test confirms that the tail element in the backing array is nil (leak fixed).
• Performance: Benchmark showed negligible overhead (~10ns/op increase from 252ns to 263ns) for the safety gain.

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PR created automatically by Jules for task 5557648803222566373 started by @arran4

[google-labs-jules]

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[arran4]

Not sure