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).

include/daedalus.h

@@ -544,6 +544,21 @@ DECLARE_QPEL_AVG(avg_mc33)
 
 #undef DECLARE_QPEL_AVG
 
+/* -------------------------------------------------------------------
+ * H.264 chroma DC 2x2 Hadamard pre-pass (per H.264 §8.5.11.1).
+ *
+ * Operates in-place on 4 int16 (the DC coefficients of an MB's
+ * chroma 4x4 AC blocks).  Pure CPU primitive — no substrate
+ * dispatch wrapper because the work is 4 adds + 4 subs.  Callers
+ * compose with QP-dependent scaling themselves; the scale shape
+ * varies by slice/PPS chroma_qp offset context.
+ *
+ * Bit-exact validated against tests/h264_chroma_dc_hadamard_ref.c
+ * (7-case spec-derived test suite including the H·H = 4·I algebraic
+ * invariant; see PR #23).
+ * ----------------------------------------------------------------- */
+void daedalus_h264_chroma_dc_hadamard_2x2(int16_t c[4]);
+
 /* -------------------------------------------------------------------
  * Recipe query — what does the API recommend for each kernel?
  * ----------------------------------------------------------------- */

src/h264_chroma_dc.c

@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: BSD-2-Clause */
+/*
+ * H.264 chroma DC 2x2 Hadamard pre-pass (public, in-tree CPU).
+ *
+ * The 4 DC coefficients of an MB's chroma 4x4 AC blocks go through
+ * this 2x2 Hadamard before quant-scaling and re-injection into the
+ * AC blocks' [0,0] coefficient.  Algorithm per H.264 §8.5.11.1.
+ *
+ * Pure CPU primitive — there's no substrate-dispatch wrapper because
+ * the work is 4 adds + 4 subs.  Callers compose with QP-dependent
+ * scaling themselves (the scale shape varies by slice/PPS chroma_qp
+ * offset context and shouldn't be baked into the kernel).
+ *
+ * Bit-exact validated against tests/h264_chroma_dc_hadamard_ref.c
+ * (7-case spec-derived test suite including the H·H = 4·I algebraic
+ * invariant; see PR #23).  Same algorithm; this is the public
+ * src-tree copy.
+ */
+#include "daedalus.h"
+
+#include <stdint.h>
+
+void daedalus_h264_chroma_dc_hadamard_2x2(int16_t c[4])
+{
+    int t0 = c[0] + c[1];
+    int t1 = c[0] - c[1];
+    int t2 = c[2] + c[3];
+    int t3 = c[2] - c[3];
+
+    c[0] = (int16_t)(t0 + t2);   /* f[0,0] = sum of all 4   */
+    c[1] = (int16_t)(t1 + t3);   /* f[0,1] = col-difference */
+    c[2] = (int16_t)(t0 - t2);   /* f[1,0] = row-difference */
+    c[3] = (int16_t)(t1 - t3);   /* f[1,1] = anti-diagonal  */
+}

tests/test_chroma_dc_hadamard.c

@@ -12,6 +12,7 @@
 #include <string.h>
 
 extern void daedalus_h264_chroma_dc_hadamard_2x2_ref(int16_t c[4]);
+extern void daedalus_h264_chroma_dc_hadamard_2x2(int16_t c[4]);  /* public API */
 
 static int check(const char *name, int16_t in[4], int16_t expect[4])
 {
@@ -112,6 +113,23 @@ int main(void)
       fail |= local_fail;
     }
 
+    /* Test 8: public API parity.  The public symbol must produce
+     * byte-identical output to the test-only ref for the same input.
+     * If the src/ Hadamard ever drifts from the spec, this catches it. */
+    {
+        int16_t input[4] = { 7, -11, 23, -42 };
+        int16_t a[4], b[4];
+        memcpy(a, input, sizeof(a));
+        memcpy(b, input, sizeof(b));
+        daedalus_h264_chroma_dc_hadamard_2x2_ref(a);
+        daedalus_h264_chroma_dc_hadamard_2x2(b);
+        int local_fail = 0;
+        for (int i = 0; i < 4; i++) if (a[i] != b[i]) local_fail = 1;
+        printf("  %-32s %s\n", "public API parity vs _ref",
+               local_fail ? "FAIL" : "PASS");
+        fail |= local_fail;
+    }
+
     if (fail == 0) printf("\nALL chroma DC Hadamard tests PASS\n");
     else           fprintf(stderr, "\n%d test(s) FAILED\n", fail);
     return fail ? 1 : 0;