v3d_runner: persistent per-pipeline command buffer

Phase 2 of the QPU-default substrate campaign — eliminate
vkAllocateCommandBuffers from the dispatch hot path.

Attaches a VkCommandBuffer to each v3d_pipeline, allocated once in
v3d_runner_create_pipeline() and freed in destroy_pipeline().  The
five dispatch_*_qpu sites switch from v3d_runner_alloc_cmdbuf() to
v3d_runner_pipeline_cmdbuf_reset() — vkResetCommandBuffer is O(1)
versus the driver-side allocation walk.  Pool was already created
with VK_COMMAND_POOL_CREATE_RESET_COMMAND_BUFFER_BIT so reset is
permitted.

Microbench (hertz, Pi 5, kernel 6.18.29, V3D 7.1):

  before (task 160 pool only):
    steady-state p50: 76.44 us
    steady-state mean: 77.95 us
  after (task 160 pool + task 161 persistent cb):
    steady-state p50: 54.56 us
    steady-state mean: 56.00 us
    -> 28% per-dispatch reduction

The remaining ~54 us steady-state is dominated by vkQueueWaitIdle +
shader execution + the two memcpy(in/out) on the dst buffer — task
162 (dmabuf import for dst) targets the memcpy half.

test_api_idct stays bit-exact across CPU/QPU/AUTO substrates.

Refs daedalus-fourier task #161.

src/v3d_runner.c

@@ -486,12 +486,27 @@ int v3d_runner_create_pipeline(v3d_runner *r, const char *spv_path,
         .pSetLayouts = &out->ds_layout,
     };
     CHK(vkAllocateDescriptorSets(r->device, &dsai, &out->desc_set));
+
+    /* Persistent command buffer — pool was created with
+     * RESET_COMMAND_BUFFER_BIT (see v3d_runner_create) so dispatch
+     * sites can call vkResetCommandBuffer on this same cb instead
+     * of paying vkAllocateCommandBuffers per call. */
+    VkCommandBufferAllocateInfo cbai = {
+        .sType = VK_STRUCTURE_TYPE_COMMAND_BUFFER_ALLOCATE_INFO,
+        .commandPool = r->pool,
+        .level = VK_COMMAND_BUFFER_LEVEL_PRIMARY,
+        .commandBufferCount = 1,
+    };
+    CHK(vkAllocateCommandBuffers(r->device, &cbai, &out->cb));
+
     return 0;
 }
 
 void v3d_runner_destroy_pipeline(v3d_runner *r, v3d_pipeline *p)
 {
     if (!p || p->pipeline == VK_NULL_HANDLE) return;
+    if (p->cb != VK_NULL_HANDLE)
+        vkFreeCommandBuffers(r->device, r->pool, 1, &p->cb);
     vkDestroyPipeline(r->device, p->pipeline, NULL);
     vkDestroyPipelineLayout(r->device, p->layout, NULL);
     vkDestroyDescriptorPool(r->device, p->pool, NULL);  /* frees its set */
@@ -499,6 +514,13 @@ void v3d_runner_destroy_pipeline(v3d_runner *r, v3d_pipeline *p)
     memset(p, 0, sizeof(*p));
 }
 
+int v3d_runner_pipeline_cmdbuf_reset(v3d_runner *r, v3d_pipeline *p)
+{
+    (void) r;
+    if (!p || p->cb == VK_NULL_HANDLE) return -1;
+    return vkResetCommandBuffer(p->cb, 0) == VK_SUCCESS ? 0 : -1;
+}
+
 int v3d_runner_bind_buffers(v3d_runner *r, v3d_pipeline *p,
                             const v3d_buffer *bufs, uint32_t n)
 {