feat(simd): Phase 2 — wire simd_nightly into crate::simd::* dispatch + matrix fix #374

Job	Run time
clippy/1.95.0	41s
hpc-stream-parallel/rayon	1m 13s
nostd/thumbv6m-none-eabi/stable	26s
native-backend/stable	7m 44s
Pass MSRV values to other jobs	2s
format/stable	21s
miri	0s
docs/nightly	0s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
cargo-careful	0s
tests/1.95.0	10m 51s
tests/beta	11m 7s
tests/stable	11m 35s
blas-msrv	1m 14s
conclusion	0s
	45m 14s

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