feat(codec): PR-X12 A2 mode bit-pack + A3-intra prediction kernel #447

Job	Run time
nostd/thumbv6m-none-eabi/stable	27s
Pass MSRV values to other jobs	3s
format/stable	21s
tier4-avx512-check	32s
clippy/1.95.0	40s
native-backend/stable	8m 18s
hpc-stream-parallel/rayon	1m 21s
docs/nightly	-1s
nightly-simd-polyfill	-1s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	-1s
cargo-careful	-1s
miri	-1s
blas-msrv	47s
tests/1.95.0	11m 4s
tests/stable	11m 25s
tests/beta	11m 26s
conclusion	0s
	46m 19s

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