Add CBWSDID covariate balancing to StackedDiD (Ustyuzhanin 2026) #2375

Job	Run time
Rust Unit Tests (ubuntu-latest)	1m 23s
Rust Unit Tests (ubuntu-24.04-arm)	1m 24s
Rust Unit Tests (windows-latest)	3m 8s
Rust Unit Tests (macos-latest)	1m 16s
Pure Python Fallback	1h 15m 15s
Python Tests (windows-latest, py3.11)	52m 59s
Python Tests (ubuntu-24.04-arm, py3.11)	40m 33s
Python Tests (ubuntu-latest, py3.11)	1h 4m 49s
Python Tests (macos-latest, py3.13)	28m 9s
Python Tests (macos-latest, py3.11)	31m 15s
Python Tests (ubuntu-24.04-arm, py3.13)	41m 56s
Python Tests (ubuntu-latest, py3.13)	1h 3m 20s
Python Tests (windows-latest, py3.14)	43m 5s
Python Tests (windows-latest, py3.13)	1h 0m 36s
Python Tests (ubuntu-latest, py3.14)	1h 13m 31s
Python Tests (ubuntu-24.04-arm, py3.14)	41m 49s
Python Tests (macos-latest, py3.14)	33m 47s
	10h 58m 15s

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