Add 3DGS HHTL CPU cascade plan #474

Job	Run time
format/stable	22s
native-backend/stable	4m 25s
hpc-stream-parallel/rayon	1m 24s
Pass MSRV values to other jobs	2s
nostd/thumbv6m-none-eabi/stable	27s
nightly-simd-polyfill	50s
clippy/1.95.0	41s
tier4-avx512-check	22s
docs/nightly	47s
miri	1m 19s
cargo-careful	44s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
blas-msrv	1m 1s
tests/beta	10m 35s
tests/stable	11m 1s
tests/1.95.0	10m 47s
conclusion	0s
	44m 47s

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