cycle 7: V3D shader for H.264 IDCT 8x8

Mirrors cycle 6 (PR #7 prior commit) but at 8x8 scale: 8 lanes per
block, 8 blocks per WG.  H.264 §8.5.13.2 1D butterfly twice (row
pass, column pass), (val + 32) >> 6 rounded + clipped + added to
dst.

Bit-exact first try on hertz (Pi 5, V3D 7.1):

  H264_IDCT4 recipe substrate:      2 (QPU)
  H264_IDCT8 recipe substrate:      2 (QPU)    ← flipped
  H264_DEBLOCK_LV recipe substrate: 2 (QPU)
  H264_QPEL_MC20 recipe substrate:  1 (CPU)    ← task #165 remaining
  H.264 IDCT 4x4: 2048/2048 bytes bit-exact
  H.264 IDCT 8x8: 2048/2048 bytes bit-exact    ← QPU
  H.264 deblock luma v: 2048/2048 bytes bit-exact
  H.264 qpel mc20: 1024/1024 bytes bit-exact

8 of 9 daedalus-fourier cycles now QPU-by-recipe.  Only cycle 9
(H.264 luma qpel mc20) still CPU — different shape (6-tap MC
filter, not a transform) so needs its own shader template; task
#165 covers it as a follow-on.

Same pattern as cycle 6 commit (65bd5c3): adds h264_idct8_pipe
field + lazy init, dispatch_h264_idct8_qpu() with 3 SSBOs,
v3d_h264_idct8.spv install rule.

Uses v3d_runner_create_buffer / destroy_buffer (will swap to
pool API once PR #6 lands).

CMakeLists.txt

@@ -295,7 +295,18 @@ 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} ${H264_IDCT4_SPV})
+    set(H264_IDCT8_SPV ${CMAKE_BINARY_DIR}/v3d_h264_idct8.spv)
+    add_custom_command(
+        OUTPUT ${H264_IDCT8_SPV}
+        COMMAND ${GLSLANG_VALIDATOR} -V --target-env vulkan1.3
+                -o ${H264_IDCT8_SPV}
+                ${CMAKE_SOURCE_DIR}/src/v3d_h264_idct8.comp
+        DEPENDS ${CMAKE_SOURCE_DIR}/src/v3d_h264_idct8.comp
+        COMMENT "glslang: v3d_h264_idct8.comp -> v3d_h264_idct8.spv"
+        VERBATIM
+    )
+
+    add_custom_target(daedalus_shaders ALL DEPENDS ${NOOP_SPV} ${IDCT8_SPV} ${LPF_SPV} ${MC_SPV} ${LPF8_SPV} ${CDEF_SPV} ${H264DEBLOCK_SPV} ${H264_IDCT4_SPV} ${H264_IDCT8_SPV})
 
     # v3d_runner — reusable Vulkan plumbing.
     add_library(v3d_runner STATIC src/v3d_runner.c)
@@ -424,6 +435,7 @@ if (DAEDALUS_BUILD_VULKAN)
         ${CDEF_SPV}
         ${H264DEBLOCK_SPV}
         ${H264_IDCT4_SPV}
+        ${H264_IDCT8_SPV}
         DESTINATION ${CMAKE_INSTALL_DATADIR}/daedalus-fourier/shaders
     )
 endif()