feat(codec): PR-X12 A2 mode bit-pack + A3-intra prediction kernel #445

Job	Run time
format/stable	21s
Pass MSRV values to other jobs	2s
nostd/thumbv6m-none-eabi/stable	27s
native-backend/stable	7m 56s
tier4-avx512-check	29s
clippy/1.95.0	41s
hpc-stream-parallel/rayon	1m 21s
miri	0s
nightly-simd-polyfill	0s
docs/nightly	0s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
cargo-careful	0s
blas-msrv	45s
tests/beta	10m 41s
tests/1.95.0	11m 19s
tests/stable	10m 44s
conclusion	0s
	44m 46s

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