h264: chroma DC 2x2 Hadamard pre-pass primitive

Adds the H.264 §8.5.11.1 chroma DC Hadamard transform.  In 4:2:0
chroma, the four DC coefficients (one from each chroma 4x4 AC block
within an MB) go through a 2x2 Hadamard before quant-scaling and
before being added back to each block's [0,0] coefficient prior to
the 4x4 AC IDCT.

This PR ships the pure Hadamard transform:

  f[0,0] = c[0,0] + c[0,1] + c[1,0] + c[1,1]
  f[0,1] = c[0,0] - c[0,1] + c[1,0] - c[1,1]
  f[1,0] = c[0,0] + c[0,1] - c[1,0] - c[1,1]
  f[1,1] = c[0,0] - c[0,1] - c[1,0] + c[1,1]

implemented as the 2-stage row+col butterfly (1:1 with the NEON
SIMD shape upstream).  Operates in-place on int16[4].

What this does NOT do (deferred to caller-side composition):

  - QP-dependent scaling per §8.5.11.2.  The scale depends on
    QP_C (with chroma_qp_offset adjustment), so the formula has
    branches (>=6 vs <6) and looks up LevelScale4x4 table values.
    The libavcodec intercept patch composes Hadamard + scale +
    shift itself since the scale shape varies by codec-level
    context (slice header chroma_qp_offset, PPS chroma_qp_offset,
    second_chroma_qp_offset for the chroma_qp_index_offset).
  - Inverse transform (decode-time used for the FORWARD direction
    is the same Hadamard up to scaling, but conceptually the spec
    distinguishes them in §8.5.11; we expose only the matrix).

Test design (tests/test_chroma_dc_hadamard.c):

  7 cases, all spec-derived hand-computations:
    - all-uniform 5 → [20, 0, 0, 0]
    - col gradient [0,10,0,10] → [20, -20, 0, 0]
    - row gradient [0,0,10,10] → [20, 0, -20, 0]
    - anti-diagonal [10,0,0,10] → [20, 0, 0, 20]
    - asymmetric [1,2,3,4] → [10, -2, -4, 0]
    - sign-alternating [-5,5,-5,5] → [0, -20, 0, 0]
    - double-Hadamard invariant: H·H = 4·I, so applying twice
      gives [4*c[0], 4*c[1], 4*c[2], 4*c[3]] for any input.

The double-Hadamard test is the strongest correctness gate: any
single sign error in the butterfly would break the H·H = 4·I
algebraic property, surfacing immediately.  All 7 PASS first try.

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

  ALL chroma DC Hadamard tests PASS

With this primitive the H.264 8-bit 4:2:0 pixel-math primitive
matrix is complete in fourier:
  - IDCT 4x4 (luma + chroma) ✓
  - IDCT 8x8 (luma, High profile) ✓
  - Chroma DC Hadamard 2x2 ✓ (this PR)
  - Deblock (8 variants) ✓
  - Intra prediction (26 modes) ✓
  - MC qpel (30 dispatches) ✓

What remains for the libavcodec intercept patch: CABAC/CAVLC entropy
decode, SPS/PPS parsing, slice header parsing, MB type / QP / CBP /
intra mode prediction.  All of that lives at the intercept layer
(it's spec-derived from the bitstream syntax, not pixel-math); the
intercept patch will call into these fourier primitives once the
metadata is decoded.

CMakeLists.txt

@@ -564,14 +564,21 @@ add_executable(test_intra_pred_chroma8x8
 target_compile_options(test_intra_pred_chroma8x8 PRIVATE -O2)
 
 # H.264 Intra_8x8 luma prediction (High profile, 9 modes + 1-2-1
-# reference-sample pre-filter).  This PR ships the pre-filter + the
-# 3 simple modes (V, H, DC); the 6 directional modes follow.
+# reference-sample pre-filter).
 add_executable(test_intra_pred_8x8_luma
     tests/test_intra_pred_8x8_luma.c
     tests/h264_intra_pred_8x8_luma_ref.c
 )
 target_compile_options(test_intra_pred_8x8_luma PRIVATE -O2)
 
+# H.264 chroma DC 2x2 Hadamard pre-pass primitive.  Pure transform,
+# no QP-dependent scaling (that's caller-side composition).
+add_executable(test_chroma_dc_hadamard
+    tests/test_chroma_dc_hadamard.c
+    tests/h264_chroma_dc_hadamard_ref.c
+)
+target_compile_options(test_chroma_dc_hadamard PRIVATE -O2)
+
 add_executable(bench_pool_overhead tests/bench_pool_overhead.c)
 target_link_libraries(bench_pool_overhead PRIVATE daedalus_core)
 target_compile_options(bench_pool_overhead PRIVATE -O2)