🎨 Improve span for flow construction #1606

Job	Run time
quality_checks_pass	1m 57s
valgrind	3m 52s
sanitize (undefined, clang)	2m 27s
build_and_test (clang, 14, 20, libc++, Debug)	2m 46s
sanitize (undefined, gcc)	3m 30s
build_and_test (clang, 15, 20, libstdc++, Debug)	2m 47s
sanitize (address, clang)	3m 13s
build_and_test (clang, 15, 20, libc++, Debug)	3m 32s
sanitize (address, gcc)	3m 30s
build_and_test (clang, 16, 20, libstdc++, Debug)	2m 56s
sanitize (thread, clang)	3m 7s
build_and_test (clang, 16, 20, libc++, Debug)	4m 23s
sanitize (thread, gcc)	3m 57s
build_and_test (clang, 17, 20, libstdc++, Debug)	3m 15s
build_and_test (clang, 17, 20, libc++, Debug)	4m 13s
build_and_test (clang, 18, 20, libstdc++, Debug)	3m 29s
build_and_test (clang, 18, 20, libc++, Debug)	4m 11s
build_and_test (gcc, 12, 20, libstdc++, Debug)	2m 48s
build_and_test (gcc, 13, 20, libstdc++, Debug)	3m 29s
merge_ok	3s
	1h 3m 25s

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