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.h

@@ -34,6 +34,12 @@ typedef struct {
     VkDescriptorSet        desc_set;
     uint32_t               n_ssbos;
     uint32_t               push_const_size;
+    /* Persistent command buffer.  Allocated at create-pipeline time;
+     * dispatch sites use v3d_runner_pipeline_cmdbuf_reset() to
+     * vkResetCommandBuffer instead of paying vkAllocateCommandBuffers
+     * per dispatch.  Pool flagged RESET_COMMAND_BUFFER_BIT so reset
+     * is permitted. */
+    VkCommandBuffer        cb;
 } v3d_pipeline;
 
 /*
@@ -121,6 +127,12 @@ int  v3d_runner_bind_buffers(v3d_runner   *r,
 /* Allocate a primary command buffer from the runner's pool. */
 VkCommandBuffer v3d_runner_alloc_cmdbuf(v3d_runner *r);
 
+/* Reset @p->cb so it can be re-recorded.  Returns 0 on success.
+ * Replaces v3d_runner_alloc_cmdbuf() on the dispatch hot path —
+ * vkResetCommandBuffer is O(1) vs vkAllocateCommandBuffers' ~1-5us
+ * driver cost. */
+int v3d_runner_pipeline_cmdbuf_reset(v3d_runner *r, v3d_pipeline *p);
+
 /* Submit `cb` to the queue and wait for completion. The classic
  * timed operation. Returns 0 on success.
  */