feat(simd): Phase 1 — explicit cargo configs + AVX2 dispatch hardening #372

Job	Run time
nostd/thumbv6m-none-eabi/stable	26s
hpc-stream-parallel/rayon	1m 32s
format/stable	23s
native-backend/stable	8m 47s
clippy/1.95.0	47s
Pass MSRV values to other jobs	3s
docs/nightly	0s
cargo-careful	-1s
miri	-1s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	-1s
tests/beta	10m 59s
blas-msrv	57s
tests/1.95.0	10m 45s
tests/stable	11m 13s
conclusion	0s
	45m 49s

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