Move ITs into their own crate #604

Job	Run time
Check formatting	11s
Run unit tests (ubuntu-latest)	48s
Run unit tests (windows-latest)	1m 37s
Check clippy	24s
Validate minimal dependency versions	25s
Run unit tests (macOS-latest)	58s
Run integration tests (5)	3m 27s
Run integration tests (2025)	3m 10s
Run integration tests (4.4)	3m 6s
Release	-1s
	14m 5s

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