Add 3DGS columnar splat codec plan #473

Job	Run time
nostd/thumbv6m-none-eabi/stable	29s
native-backend/stable	8m 24s
hpc-stream-parallel/rayon	1m 24s
nightly-simd-polyfill	49s
Pass MSRV values to other jobs	2s
tier4-avx512-check	27s
clippy/1.95.0	39s
format/stable	21s
docs/nightly	49s
miri	1m 13s
cargo-careful	44s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
blas-msrv	49s
tests/stable	10m 42s
tests/1.95.0	11m 23s
tests/beta	11m 31s
conclusion	-1s
	49m 45s

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