draft: MRTR (SEP-2322) lowlevel plumbing + handler-shape comparison #1558

Job	Run time
checks / readme-snippets	13s
checks / pre-commit	34s
checks / test (3.11, lowest-direct, ubuntu-latest)	52s
checks / test (3.11, lowest-direct, windows-latest)	2m 22s
checks / test (3.10, lowest-direct, windows-latest)	2m 42s
checks / test (3.12, lowest-direct, windows-latest)	4m 14s
checks / test (3.10, locked, ubuntu-latest)	51s
checks / test (3.10, locked, windows-latest)	2m 14s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 16s
checks / test (3.11, locked, windows-latest)	2m 32s
checks / test (3.10, lowest-direct, ubuntu-latest)	57s
checks / test (3.13, locked, windows-latest)	2m 15s
checks / test (3.11, locked, ubuntu-latest)	50s
checks / test (3.12, locked, windows-latest)	3m 48s
checks / test (3.14, locked, ubuntu-latest)	2m 12s
checks / test (3.13, lowest-direct, windows-latest)	2m 46s
checks / test (3.13, lowest-direct, ubuntu-latest)	54s
checks / test (3.14, lowest-direct, windows-latest)	2m 21s
checks / test (3.14, locked, windows-latest)	3m 21s
checks / test (3.12, lowest-direct, ubuntu-latest)	56s
checks / test (3.13, locked, ubuntu-latest)	53s
checks / test (3.12, locked, ubuntu-latest)	53s
all-green	3s
	39m 59s

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