h264: V3D shader for qpel mc02 (vertical half-pel)

Sibling of cycle 9's v3d_h264_qpel_mc20.comp.  Same 6-tap H.264 luma
half-pel filter, transposed to vertical orientation: filter reads
rows [-2..+3] of source per output pixel instead of cols.

Shader is ~58 lines (vs mc20's 86) — same WG geometry (64 lanes /
1 block per WG / 1 lane per output pixel).  The address arithmetic
flips: row_base = src_off + r*stride + c (mc20) → col_base =
src_off + c, then col_base + (r±N)*stride (mc02).

dispatch_h264_qpel_mc02_qpu mirrors the mc20 QPU dispatch; src_max
calculation differs since the V kernel reads rows -2..+10 of source
(13 rows × stride wide) vs mc20's cols -2..+10 (8 rows × stride+11).
For 8x8 blocks: src_max = src_off + 10*stride + 8.

Recipe table flips DAEDALUS_KERNEL_H264_QPEL_MC02 from CPU to QPU.

Verified on hertz:

  $ ./build/test_api_h264 | grep qpel
    H.264 qpel mc20: 1024/1024 bytes bit-exact (100.0000%)
    H.264 qpel mc02: 2048/2048 bytes bit-exact (100.0000%)

QPU coverage for the 30 qpel positions:
  put_  mc20 ✓ (cycle 9)   mc02 ✓ (this PR)
        all 13 other put_  CPU NEON
  avg_  all 15 positions   CPU NEON

Next-priority candidates by per-frame impact (per PR #24 bench):
  mc22 (2D half-pel)  — 71.5 ns/block NEON × 32 640 blocks worst
                        case = 2.33 ms/frame at 1080p.  Most-used
                        qpel position in real H.264 streams.
  mc11/mc13/mc31/mc33 — corner ¼-pel positions, structurally similar
                        to mc20 + mc02 with L2 averaging.

The cascaded H+V structure of mc22 means it can either share the
existing mc20 + mc02 shaders' L2 (compute mc20 into tmp, then mc02
on tmp) or get a dedicated 2-stage pipeline.  Follow-up.

CMakeLists.txt

@@ -350,7 +350,18 @@ if (DAEDALUS_BUILD_VULKAN)
         VERBATIM
     )
 
-    add_custom_target(daedalus_shaders ALL DEPENDS ${NOOP_SPV} ${IDCT8_SPV} ${LPF_SPV} ${MC_SPV} ${LPF8_SPV} ${CDEF_SPV} ${H264DEBLOCK_SPV} ${H264DEBLOCK_H_SPV} ${H264DEBLOCK_CHROMA_V_SPV} ${H264DEBLOCK_CHROMA_H_SPV} ${H264_IDCT4_SPV} ${H264_IDCT8_SPV} ${H264_QPEL_MC20_SPV})
+    set(H264_QPEL_MC02_SPV ${CMAKE_BINARY_DIR}/v3d_h264_qpel_mc02.spv)
+    add_custom_command(
+        OUTPUT ${H264_QPEL_MC02_SPV}
+        COMMAND ${GLSLANG_VALIDATOR} -V --target-env vulkan1.3
+                -o ${H264_QPEL_MC02_SPV}
+                ${CMAKE_SOURCE_DIR}/src/v3d_h264_qpel_mc02.comp
+        DEPENDS ${CMAKE_SOURCE_DIR}/src/v3d_h264_qpel_mc02.comp
+        COMMENT "glslang: v3d_h264_qpel_mc02.comp -> v3d_h264_qpel_mc02.spv"
+        VERBATIM
+    )
+
+    add_custom_target(daedalus_shaders ALL DEPENDS ${NOOP_SPV} ${IDCT8_SPV} ${LPF_SPV} ${MC_SPV} ${LPF8_SPV} ${CDEF_SPV} ${H264DEBLOCK_SPV} ${H264DEBLOCK_H_SPV} ${H264DEBLOCK_CHROMA_V_SPV} ${H264DEBLOCK_CHROMA_H_SPV} ${H264_IDCT4_SPV} ${H264_IDCT8_SPV} ${H264_QPEL_MC20_SPV} ${H264_QPEL_MC02_SPV})
 
     # v3d_runner — reusable Vulkan plumbing.
     add_library(v3d_runner STATIC src/v3d_runner.c)
@@ -489,6 +500,7 @@ if (DAEDALUS_BUILD_VULKAN)
         ${H264_IDCT4_SPV}
         ${H264_IDCT8_SPV}
         ${H264_QPEL_MC20_SPV}
+        ${H264_QPEL_MC02_SPV}
         DESTINATION ${CMAKE_INSTALL_DATADIR}/daedalus-fourier/shaders
     )
 endif()