Stronger, clearer test suite with cross-engine verification #415

Job	Run time
env-prepare	4s
source-test (macos-15)	13m 29s
source-test (ubuntu-24.04)	15m 3s
source-test (windows-2022)	24m 29s
package-test (macos-15)	10m 3s
package-test (ubuntu-24.04)	14m 42s
package-test (windows-2022)	23m 33s
run-reusable-workflows / dashboard	2m 22s
	1h 43m 45s

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