feat: ServerRunner — per-connection orchestrator #1939

Job	Run time
checks / readme-snippets	12s
checks / pre-commit	40s
checks / test (3.11, locked, ubuntu-latest)	44s
checks / test (3.11, lowest-direct, windows-latest)	2m 37s
checks / test (3.12, lowest-direct, ubuntu-latest)	50s
checks / test (3.10, lowest-direct, windows-latest)	2m 50s
checks / test (3.10, locked, windows-latest)	3m 1s
checks / test (3.11, lowest-direct, ubuntu-latest)	49s
checks / test (3.11, locked, windows-latest)	2m 20s
checks / test (3.10, lowest-direct, ubuntu-latest)	49s
checks / test (3.13, lowest-direct, ubuntu-latest)	48s
checks / test (3.12, locked, windows-latest)	3m 20s
checks / test (3.12, locked, ubuntu-latest)	53s
checks / test (3.12, lowest-direct, windows-latest)	4m 36s
checks / test (3.10, locked, ubuntu-latest)	45s
checks / test (3.13, locked, windows-latest)	2m 9s
checks / test (3.14, lowest-direct, windows-latest)	2m 26s
checks / test (3.14, locked, windows-latest)	3m 34s
checks / test (3.14, locked, ubuntu-latest)	2m 20s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 15s
checks / test (3.13, lowest-direct, windows-latest)	2m 42s
checks / test (3.13, locked, ubuntu-latest)	45s
all-green	4s
	40m 29s

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