feat(simd): Phase 2 — wire simd_nightly into crate::simd::* dispatch + matrix fix #377

Job	Run time
nostd/thumbv6m-none-eabi/stable	26s
Pass MSRV values to other jobs	2s
clippy/1.95.0	42s
format/stable	21s
hpc-stream-parallel/rayon	1m 31s
native-backend/stable	7m 49s
docs/nightly	0s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
cargo-careful	0s
miri	0s
tests/1.95.0	11m 12s
tests/stable	11m 37s
blas-msrv	2m 38s
tests/beta	11m 48s
conclusion	-1s
	48m 5s

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