h264: expose chroma DC 2x2 Hadamard as public API

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).

CMakeLists.txt

@@ -391,6 +391,7 @@ endif()
 
 add_library(daedalus_core STATIC
     src/daedalus_core.c
+    src/h264_chroma_dc.c
     src/v3d_runner.c
     ${FFASM_SOURCES}
     ${FFASM_LPF_SOURCES}
@@ -577,6 +578,9 @@ add_executable(test_chroma_dc_hadamard
     tests/test_chroma_dc_hadamard.c
     tests/h264_chroma_dc_hadamard_ref.c
 )
+# Links daedalus_core to pull in the public daedalus_h264_chroma_dc_hadamard_2x2
+# symbol (for the public-API parity test added in this PR).
+target_link_libraries(test_chroma_dc_hadamard PRIVATE daedalus_core)
 target_compile_options(test_chroma_dc_hadamard PRIVATE -O2)
 
 # H.264 primitives latency benchmark (NEON CPU baseline).