PR #23 added the Hadamard as a test-only spec reference; this PR
promotes it to a public symbol in src/ so consumers (the eventual
marfrit-packages substitution-arc patch 0011) can link against it.
New: void daedalus_h264_chroma_dc_hadamard_2x2(int16_t c[4]);
— operates in-place on 4 int16, no QP-dependent scaling
(caller composes that themselves per §8.5.11.2).
The src/ implementation is byte-for-byte identical to the test-only
ref in tests/h264_chroma_dc_hadamard_ref.c (kept as a separate
spec-validation copy). A new "public API parity" test case verifies
the two produce identical output for a non-trivial input.
Pure CPU primitive — no substrate-dispatch wrapper because the work
is 4 adds + 4 subs; the substrate machinery would cost more than
the kernel itself.
Verified on hertz:
$ ./build/test_chroma_dc_hadamard
all-uniform 5 PASS
col gradient [0,10,0,10] PASS
row gradient [0,0,10,10] PASS
anti-diagonal [10,0,0,10] PASS
asymmetric [1,2,3,4] PASS
sign-alternating [-5,5,-5,5] PASS
double-Hadamard = 4*orig PASS
public API parity vs _ref PASS
ALL chroma DC Hadamard tests PASS
$ nm -g build/libdaedalus_core.a | grep chroma_dc_hadamard
0000000000000000 T daedalus_h264_chroma_dc_hadamard_2x2
Unblocks marfrit-packages 0011 (substituting
H264DSPContext.chroma_dc_dequant_idct, which composes the Hadamard
+ qmul scaling).
claude-noether