feat(codec): PR-X12 v1 — λ-RDO mode selection + rANS entropy coder #531

Job	Run time
clippy/1.95.0	45s
Pass MSRV values to other jobs	4s
format/stable	22s
hpc-stream-parallel/rayon	1m 21s
native-backend/stable	8m 4s
nostd/thumbv6m-none-eabi/stable	25s
tier4-avx512-check	30s
nightly-simd-polyfill	0s
cargo-careful	0s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
miri	-1s
docs/nightly	-1s
tests/beta	11m 24s
blas-msrv	1m 6s
tests/1.95.0	11m 24s
tests/stable	10m 50s
conclusion	0s
	46m 13s

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