draft: MRTR (SEP-2322) lowlevel plumbing + handler-shape comparison #1559

Job	Run time
checks / pre-commit	32s
checks / readme-snippets	15s
checks / test (3.10, lowest-direct, ubuntu-latest)	57s
checks / test (3.11, lowest-direct, ubuntu-latest)	59s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 13s
checks / test (3.11, lowest-direct, windows-latest)	2m 36s
checks / test (3.10, locked, ubuntu-latest)	50s
checks / test (3.10, lowest-direct, windows-latest)	2m 59s
checks / test (3.13, lowest-direct, ubuntu-latest)	53s
checks / test (3.12, lowest-direct, ubuntu-latest)	57s
checks / test (3.13, locked, ubuntu-latest)	47s
checks / test (3.14, lowest-direct, windows-latest)	3m 32s
checks / test (3.10, locked, windows-latest)	2m 50s
checks / test (3.13, lowest-direct, windows-latest)	2m 55s
checks / test (3.12, lowest-direct, windows-latest)	3m 27s
checks / test (3.13, locked, windows-latest)	3m 10s
checks / test (3.14, locked, windows-latest)	2m 42s
checks / test (3.14, locked, ubuntu-latest)	2m 18s
checks / test (3.12, locked, ubuntu-latest)	56s
checks / test (3.12, locked, windows-latest)	3m 6s
checks / test (3.11, locked, ubuntu-latest)	50s
checks / test (3.11, locked, windows-latest)	2m 6s
all-green	4s
	40m 54s

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