feat(codec): PR-X12 v1 — λ-RDO mode selection + rANS entropy coder #529

Job	Run time
format/stable	21s
clippy/1.95.0	39s
hpc-stream-parallel/rayon	1m 27s
Pass MSRV values to other jobs	3s
nostd/thumbv6m-none-eabi/stable	26s
native-backend/stable	7m 53s
tier4-avx512-check	29s
miri	-1s
cargo-careful	-1s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	-1s
nightly-simd-polyfill	-1s
docs/nightly	-1s
tests/stable	10m 44s
tests/1.95.0	11m 27s
tests/beta	11m 36s
blas-msrv	50s
conclusion	-1s
	45m 49s

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