v3d_runner: buffer pool for QPU dispatch hot path

Per the QPU-default substrate decree 2026-05-23: the per-dispatch
vkAllocateMemory in dispatch_*_qpu was the biggest single fixable
contributor to QPU dispatch overhead.  This pools v3d_buffer
allocations by power-of-2 size class so the second-and-subsequent
dispatch hits a freelist instead of paying ~10-50us of Mesa-V3D7
memory-allocation cost per call.

API additions (v3d_runner.h):
  - v3d_runner_acquire_buffer(): pulls from per-bucket freelist;
    falls through to v3d_runner_create_buffer() on miss.
  - v3d_runner_release_buffer(): pushes back onto the freelist; the
    backing VkBuffer/VkDeviceMemory only get vkFreeMemory'd in
    v3d_runner_destroy().
  - v3d_runner_pool_total_bytes(): diagnostic watermark.

Size classes 2^8..2^23 (256 B to 8 MiB).  Oversize requests fall
through to non-pooled (vkAllocateMemory) for both ends — pool stays
correct, just degenerates to old behaviour for those calls.

Migration: daedalus_core.c dispatch_*_qpu paths globally swap
create_buffer → acquire_buffer and destroy_buffer → release_buffer.
All five QPU dispatch functions (idct8 / lpf / mc_8h / cdef /
h264_deblock) now reuse buffers across calls.  test_api_idct stays
bit-exact (4096/4096 bytes on CPU/QPU/AUTO substrates on hertz).

Microbench (tests/bench_pool_overhead.c) on hertz (Pi 5,
6.18.29+rpt-rpi-2712, V3D 7.1):

  call 0:  434.89 us  (cold — 3x vkAllocateMemory)
  call 1:  100.06 us  (pool hit on all 3 buffers)
  steady-state:
    p50:    76.44 us
    p90:    90.52 us
    mean:   77.95 us
  first-call / steady-state ratio: 5.7x

The remaining ~76us steady-state is dominated by vkQueueWaitIdle +
shader execution + per-call descriptor-set update + command-buffer
allocation — addressed in follow-on tasks 161 (persistent cmdbuf)
and 162 (dmabuf import for dst, eliminates memcpy in/out).

Refs daedalus-fourier task #160.

CMakeLists.txt

@@ -492,6 +492,10 @@ add_executable(test_api_opportunistic_qpu tests/test_api_opportunistic_qpu.c)
 target_link_libraries(test_api_opportunistic_qpu PRIVATE daedalus_core)
 target_compile_options(test_api_opportunistic_qpu PRIVATE -O2)
 
+add_executable(bench_pool_overhead tests/bench_pool_overhead.c)
+target_link_libraries(bench_pool_overhead PRIVATE daedalus_core)
+target_compile_options(bench_pool_overhead PRIVATE -O2)
+
 if (DAEDALUS_BUILD_VULKAN)
 # (re-open the conditional so the closing endif() below balances)