Harden backend boundary follow-up #250

Job	Run time
Format, lint, test, and audit	1m 53s
Platform tests (macos-latest)	43s
Miri	19m 7s
Platform tests (macos-15-intel)	1m 7s
no_std target build (x86_64-unknown-freebsd)	18s
no_std target build (wasm32-unknown-unknown)	21s
Platform tests (windows-latest)	50s
Platform tests (ubuntu-latest)	32s
Platform tests (macos-26)	27s
no_std target build (thumbv7em-none-eabihf)	19s
no_std target build (aarch64-unknown-linux-gnu)	20s
Platform tests (macos-15)	31s
no_std target build (x86_64-unknown-linux-gnu)	22s
	26m 50s

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