perf(spillover): sparse cKDTree branch for staggered nearest-treated … #2882

Job	Run time
Rust Unit Tests (macos-latest)	1m 49s
Rust Unit Tests (ubuntu-24.04-arm)	1m 57s
Pure Python Fallback	1h 32m 51s
Rust Unit Tests (windows-latest)	2m 34s
Python Tests (macos-latest, py3.14)	36m 56s
Python Tests (windows-latest, py3.11)	59m 57s
Python Tests (macos-latest, py3.11)	38m 54s
Rust Unit Tests (ubuntu-latest)	1m 27s
Python Tests (ubuntu-latest, py3.14)	58m 48s
Python Tests (ubuntu-24.04-arm, py3.13)	32m 6s
Python Tests (ubuntu-24.04-arm, py3.14)	31m 27s
Python Tests (macos-latest, py3.13)	38m 2s
Python Tests (windows-latest, py3.14)	1h 1m 15s
Python Tests (windows-latest, py3.13)	55m 27s
Python Tests (ubuntu-latest, py3.13)	58m 48s
Python Tests (ubuntu-latest, py3.11)	58m 44s
Python Tests (ubuntu-24.04-arm, py3.11)	33m 7s
	11h 4m 9s

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