draft: MRTR (SEP-2322) lowlevel plumbing + handler-shape comparison #1556

Job	Run time
checks / pre-commit	33s
checks / readme-snippets	11s
checks / test (3.11, lowest-direct, ubuntu-latest)	58s
checks / test (3.10, lowest-direct, windows-latest)	2m 24s
checks / test (3.11, lowest-direct, windows-latest)	2m 54s
checks / test (3.10, lowest-direct, ubuntu-latest)	55s
checks / test (3.10, locked, ubuntu-latest)	51s
checks / test (3.10, locked, windows-latest)	2m 19s
checks / test (3.11, locked, ubuntu-latest)	51s
checks / test (3.12, locked, ubuntu-latest)	54s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 16s
checks / test (3.12, lowest-direct, windows-latest)	3m 32s
checks / test (3.14, locked, ubuntu-latest)	2m 14s
checks / test (3.11, locked, windows-latest)	2m 12s
checks / test (3.13, locked, ubuntu-latest)	50s
checks / test (3.12, locked, windows-latest)	3m 7s
checks / test (3.13, locked, windows-latest)	1m 57s
checks / test (3.14, lowest-direct, windows-latest)	2m 13s
checks / test (3.13, lowest-direct, windows-latest)	2m 33s
checks / test (3.14, locked, windows-latest)	2m 58s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.13, lowest-direct, ubuntu-latest)	51s
all-green	5s
	37m 38s

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