De-flake conformance CI: solo re-verification, spawn-storm reduction, result artifacts #3305

Job	Run time
checks / readme-snippets	15s
checks / docs	38s
checks / pre-commit	50s
checks / test (3.10, locked, windows-latest)	1m 55s
checks / test (3.11, lowest-direct, windows-latest)	2m 2s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 25s
checks / test (3.11, locked, ubuntu-latest)	1m 16s
checks / test (3.10, lowest-direct, windows-latest)	1m 57s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 12s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 26s
checks / test (3.10, locked, ubuntu-latest)	1m 30s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 10s
checks / test (3.12, locked, windows-latest)	1m 52s
checks / test (3.11, locked, windows-latest)	1m 57s
checks / test (3.13, locked, windows-latest)	1m 58s
checks / test (3.13, locked, ubuntu-latest)	1m 6s
checks / test (3.14, locked, ubuntu-latest)	2m 17s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 17s
checks / test (3.12, lowest-direct, windows-latest)	2m 2s
checks / test (3.14, locked, windows-latest)	2m 43s
checks / test (3.13, lowest-direct, windows-latest)	1m 36s
checks / test (3.14, lowest-direct, windows-latest)	1m 43s
checks / test (3.12, locked, ubuntu-latest)	1m 36s
all-green	4s
	35m 47s

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