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).

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;