cycle 6: V3D shader for H.264 IDCT 4x4 (first cycle-6 QPU dispatch)

Per the QPU-default substrate decree 2026-05-23, cycle 6 (H.264
IDCT 4x4 + add) was the highest-priority H.264 kernel to flip
from NEON-only to QPU-capable.  The same shape as VP9 IDCT 8x8
(cycle 1) — two-pass butterfly with shared-memory transpose —
but at 4x4 scale: 4 lanes per block, 16 blocks per WG.

What's added:

  - src/v3d_h264_idct4.comp: GLSL compute shader implementing
    the H.264 §8.5.12.1 1D butterfly twice (row pass then column
    pass), with (val + 32) >> 6 rounding and clip-to-u8 add to
    dst.  Block memory layout is column-major (matches FFmpeg
    `ff_h264_idct_add_neon` convention).

  - CMakeLists: glslang rule + install entry for v3d_h264_idct4.spv.

  - dispatch_h264_idct4_qpu() in daedalus_core.c: lazy pipeline
    init, 3 SSBOs (coeffs / dst / meta as uvec4), push-constant
    (n_blocks, dst_stride), 16 blocks per WG dispatch.  Matches
    the existing dispatch_*_qpu patterns; uses
    v3d_runner_create_buffer / destroy_buffer (will swap to
    pool API once PR #6 lands).

  - daedalus_dispatch_h264_idct4() replaces ROUTE_CPU_ONLY with
    the same CPU/QPU substrate switch the deblock dispatch uses.

  - daedalus_recipe_substrate_for(H264_IDCT4) returns QPU now
    that the shader exists.

Verification on hertz (Pi 5 + V3D 7.1):

  $ ./test_api_h264
  === Phase 8a API smoke: H.264 kernels via recipe dispatch ===
    H264_IDCT4 recipe substrate:      2 (1=CPU, 2=QPU)
    H264_IDCT8 recipe substrate:      1
    H264_DEBLOCK_LV recipe substrate: 2
    H264_QPEL_MC20 recipe substrate:  1
    H.264 IDCT 4x4: 2048/2048 bytes bit-exact (100.0000%)   ← QPU
    H.264 IDCT 8x8: 2048/2048 bytes bit-exact
    H.264 deblock luma v: 2048/2048 bytes bit-exact
    H.264 qpel mc20: 1024/1024 bytes bit-exact

The AUTO-substrate path now picks QPU for H.264 IDCT 4x4, and
the output is bit-exact against the C reference (which is
identical to the NEON .S code by construction — same FFmpeg
upstream).

Remaining cycle-6/7/9 work in task #165:
  - cycle 7: H.264 IDCT 8x8 (template same shape; 8 lanes per
    block, fewer blocks per WG)
  - cycle 9: H.264 luma qpel mc20 (different shape — 6-tap MC
    not a transform)

This commit lands the cycle-6 piece of task #165.

CMakeLists.txt

@@ -284,7 +284,18 @@ 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})
+    set(H264_IDCT4_SPV ${CMAKE_BINARY_DIR}/v3d_h264_idct4.spv)
+    add_custom_command(
+        OUTPUT ${H264_IDCT4_SPV}
+        COMMAND ${GLSLANG_VALIDATOR} -V --target-env vulkan1.3
+                -o ${H264_IDCT4_SPV}
+                ${CMAKE_SOURCE_DIR}/src/v3d_h264_idct4.comp
+        DEPENDS ${CMAKE_SOURCE_DIR}/src/v3d_h264_idct4.comp
+        COMMENT "glslang: v3d_h264_idct4.comp -> v3d_h264_idct4.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})
 
     # v3d_runner — reusable Vulkan plumbing.
     add_library(v3d_runner STATIC src/v3d_runner.c)
@@ -412,6 +423,7 @@ if (DAEDALUS_BUILD_VULKAN)
         ${LPF8_SPV}
         ${CDEF_SPV}
         ${H264DEBLOCK_SPV}
+        ${H264_IDCT4_SPV}
         DESTINATION ${CMAKE_INSTALL_DATADIR}/daedalus-fourier/shaders
     )
 endif()