Phase 8c: H.264 luma qpel mc20 through public API

Extends daedalus-fourier with daedalus_recipe_dispatch_h264_qpel_mc20
so libavcodec.so can route H264QpelContext.put_h264_qpel_pixels_tab[1][2]
through the recipe layer instead of ff_put_h264_qpel8_mc20_neon directly.

API additions (header + library):
  - daedalus_h264_qpel_meta { dst_off, src_off }
  - daedalus_dispatch_h264_qpel_mc20(ctx, sub, dst, src, stride,
                                     n_blocks, meta)
  - daedalus_recipe_dispatch_h264_qpel_mc20(...)  (AUTO wrapper)
  - DAEDALUS_KERNEL_H264_QPEL_MC20 = 9 in the recipe-query enum
  - daedalus_recipe_substrate_for() returns CPU NEON for cycle 9

The 6-tap horizontal half-pel filter signature matches FFmpeg's
H264QpelContext convention exactly: dst and src share a single stride
and src already points at output column 0 (filter reads cols -2..+3).
Single-stride API to make the marfrit-packages FFmpeg shim a
straight pointer-pass; no buffer rearrangement.

Verdict per docs/k9_h264qpel_mc20.md: CPU NEON.  Per-block 7.6 ns
gives 135x margin over 30 fps 1080p; QPU dispatch floor at ~250 ns
makes any V3D shader strictly worse.  Recipe table reflects that —
the recipe_dispatch entry is a one-line forward to the CPU path.

CMakeLists changes:
  - h264qpel_neon.S added to the daedalus_core static lib (only the
    bench targets owned it before; now the public API needs it too)
  - tests/h264_qpel8_mc20_ref.c added to the test_api_h264 target

Phase 8a/8b smoke gains a 4th case (test_qpel_mc20): 1024/1024
bytes bit-exact via daedalus_recipe_dispatch_h264_qpel_mc20.

Refs reauktion/daedalus-v4l2#11 — substitution arc step 2 cycle 9.

tests/test_api_h264.c

@@ -18,6 +18,8 @@ extern void daedalus_h264_idct_add_ref(uint8_t *dst, int16_t *block, ptrdiff_t s
 extern void daedalus_h264_idct8_add_ref(uint8_t *dst, int16_t *block, ptrdiff_t stride);
 extern void daedalus_h264_v_loop_filter_luma_ref(uint8_t *pix, ptrdiff_t stride,
                                                   int alpha, int beta, int8_t tc0[4]);
+extern void daedalus_put_h264_qpel8_mc20_ref(uint8_t *dst, const uint8_t *src,
+                                              ptrdiff_t stride);
 
 static uint64_t xs_state = 0xa11264ULL;
 static inline uint64_t xs(void) {
@@ -143,6 +145,46 @@ static int test_deblock(void)
     return diff == 0 ? 0 : 1;
 }
 
+static int test_qpel_mc20(void)
+{
+    /* Cycle 9 — one 8x8 block per 16-wide row-tile, 8 tiles. Each tile
+     * holds rows 0..7; src[c-2..c+3] read via SRC_COL offset matches the
+     * cycle-9 bench convention so the same C reference and NEON .S can
+     * be compared. */
+    enum { N = 8, TILE_STRIDE = 16, TILE_ROWS = 8,
+           TILE_BYTES = TILE_ROWS * TILE_STRIDE, TOTAL = N * TILE_BYTES,
+           SRC_COL = 3 };
+    daedalus_ctx *ctx = daedalus_ctx_create();
+    if (!ctx) return 1;
+
+    uint8_t src[TOTAL], dst[TOTAL], dst_ref[TOTAL];
+    daedalus_h264_qpel_meta meta[N];
+
+    for (int i = 0; i < TOTAL; i++) src[i] = (uint8_t)(xs() & 0xff);
+    memset(dst, 0, sizeof(dst));
+    memset(dst_ref, 0, sizeof(dst_ref));
+
+    for (int i = 0; i < N; i++) {
+        meta[i].src_off = (uint32_t)(i * TILE_BYTES + SRC_COL);
+        meta[i].dst_off = (uint32_t)(i * TILE_BYTES + SRC_COL);
+    }
+
+    for (int i = 0; i < N; i++)
+        daedalus_put_h264_qpel8_mc20_ref(dst_ref + meta[i].dst_off,
+                                          src + meta[i].src_off,
+                                          TILE_STRIDE);
+
+    int rc = daedalus_recipe_dispatch_h264_qpel_mc20(ctx, dst, src,
+                                                      TILE_STRIDE, N, meta);
+    if (rc) { fprintf(stderr, "qpel_mc20 dispatch rc=%d\n", rc); return 1; }
+    int diff = 0;
+    for (int i = 0; i < TOTAL; i++) if (dst[i] != dst_ref[i]) diff++;
+    printf("  H.264 qpel mc20: %d/%d bytes bit-exact (%.4f%%)\n",
+           TOTAL - diff, TOTAL, 100.0 * (TOTAL - diff) / TOTAL);
+    daedalus_ctx_destroy(ctx);
+    return diff == 0 ? 0 : 1;
+}
+
 int main(void)
 {
     printf("=== Phase 8a API smoke: H.264 kernels via recipe dispatch ===\n");
@@ -152,10 +194,13 @@ int main(void)
            (int) daedalus_recipe_substrate_for(DAEDALUS_KERNEL_H264_IDCT8));
     printf("  H264_DEBLOCK_LV recipe substrate: %d\n",
            (int) daedalus_recipe_substrate_for(DAEDALUS_KERNEL_H264_DEBLOCK_LV));
+    printf("  H264_QPEL_MC20 recipe substrate:  %d\n",
+           (int) daedalus_recipe_substrate_for(DAEDALUS_KERNEL_H264_QPEL_MC20));
 
     int fail = 0;
     fail |= test_idct4();
     fail |= test_idct8();
     fail |= test_deblock();
+    fail |= test_qpel_mc20();
     return fail;
 }