Fix SIMD prototype block staging #237

Job	Run time
Format, lint, test, and audit	1m 33s
Miri	15m 21s
no_std target build (x86_64-unknown-freebsd)	21s
no_std target build (thumbv7em-none-eabihf)	17s
Platform tests (macos-15)	29s
Platform tests (macos-15-intel)	1m 46s
Platform tests (macos-26)	29s
Platform tests (ubuntu-latest)	32s
no_std target build (x86_64-unknown-linux-gnu)	18s
Platform tests (windows-latest)	57s
no_std target build (aarch64-unknown-linux-gnu)	18s
Platform tests (macos-latest)	28s
no_std target build (wasm32-unknown-unknown)	20s
	23m 9s

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