h264: V3D shader for qpel mc22 (2D half-pel "j" position)

Cascaded H+V 6-tap filter per H.264 §8.4.2.2.1.  Highest per-frame
impact among missing qpel positions (PR #24 bench: 71.5 ns/block
NEON, 2.33 ms/frame worst-case all-mc22 at 1080p).

Per-lane structure: each lane runs the FULL cascade independently —
computes 6 horizontal lowpass int16 intermediates at rows r-2..r+3
of its column, then a vertical lowpass on those with +512 >> 10
final scale.  ~50 ALU ops per lane.

Design choice: NO shared memory / barriers.  Alternative was to
cache the h-lowpass intermediates in shared memory (13 rows × 8 cols
of int16 per WG), trading shared-memory bank pressure + a barrier
for ~6× less h-lowpass compute.  V3D L2 absorbs the redundant src
reads across lanes; the per-lane compute is cheap (multiply-add ALU
units idle anyway during dst write).  Simpler shader, fewer SPIR-V
ops, easier to extend to mc12/mc21/etc. later.

CANNOT just cascade mc20 → mc02 because the intermediate must be
int16 (no per-stage clip): the +512 >> 10 final scale assumes both
6-tap scalings preserved through the pipeline.  Dedicated kernel.

dispatch_h264_qpel_mc22_qpu mirrors the existing mc20/mc02 shape;
src_max = src_off + 10*stride + 11 covers both the V (rows -2..+10)
and H (cols -2..+10) read windows in one bound.

Recipe table flips DAEDALUS_KERNEL_H264_QPEL_MC22 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%)
    H.264 qpel mc22: 2048/2048 bytes bit-exact (100.0000%)

Qpel QPU coverage now: 3 anchors (mc20 H, mc02 V, mc22 HV) — these
are the half-pel "building blocks" the 12 other qpel positions
combine via L2 averaging.  Remaining variants (quarter-pel singles
mc01/03/10/30 and the 8 diagonals) can dispatch through the existing
shaders + a small L2-averaging compose step, or get dedicated kernels.

CMakeLists.txt

@@ -361,7 +361,18 @@ 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} ${H264_QPEL_MC02_SPV})
+    set(H264_QPEL_MC22_SPV ${CMAKE_BINARY_DIR}/v3d_h264_qpel_mc22.spv)
+    add_custom_command(
+        OUTPUT ${H264_QPEL_MC22_SPV}
+        COMMAND ${GLSLANG_VALIDATOR} -V --target-env vulkan1.3
+                -o ${H264_QPEL_MC22_SPV}
+                ${CMAKE_SOURCE_DIR}/src/v3d_h264_qpel_mc22.comp
+        DEPENDS ${CMAKE_SOURCE_DIR}/src/v3d_h264_qpel_mc22.comp
+        COMMENT "glslang: v3d_h264_qpel_mc22.comp -> v3d_h264_qpel_mc22.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} ${H264_QPEL_MC22_SPV})
 
     # v3d_runner — reusable Vulkan plumbing.
     add_library(v3d_runner STATIC src/v3d_runner.c)
@@ -501,6 +512,7 @@ if (DAEDALUS_BUILD_VULKAN)
         ${H264_IDCT8_SPV}
         ${H264_QPEL_MC20_SPV}
         ${H264_QPEL_MC02_SPV}
+        ${H264_QPEL_MC22_SPV}
         DESTINATION ${CMAKE_INSTALL_DATADIR}/daedalus-fourier/shaders
     )
 endif()