fix: wire atom_stats_buf + q_var to populate_q_out — C51 atom utilization was always 0%

compute_expected_q was called with null_atom_stats=0 (NULL pointer),
so the kernel skipped atom entropy/utilization accumulation entirely.
atom_stats_buf existed but was never passed. This disabled:
- G4 adaptive gamma annealing (uses atom_utilization)
- Homeostatic regularizer atom_util observable
- ISV atom utilization signal

Now passes atom_stats_buf.raw_ptr() with a cuMemsetD32 zero before
each call (kernel uses atomicAdd). Also passes q_var_buf_trainer for
per-action Q-variance computation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/ml/src/cuda_pipeline/gpu_dqn_trainer.rs

@@ -7945,9 +7945,13 @@ impl GpuDqnTrainer {
         let block_dim = 256_u32;
         let grid_dim = ((batch_size as u32 + block_dim - 1) / block_dim).max(1);
         let q_out_ptr = self.q_out_buf.raw_ptr();
-        let null_atom_stats = 0u64;
-        let null_q_var = 0u64;
+        let atom_stats_ptr = self.atom_stats_buf.raw_ptr();
+        let q_var_ptr = self.q_var_buf_trainer.raw_ptr();
         let atom_positions_buf_ptr = self.atom_positions_buf.raw_ptr();
+        // Zero atom_stats before accumulation (kernel uses atomicAdd)
+        unsafe {
+            cudarc::driver::sys::cuMemsetD32Async(atom_stats_ptr, 0, 2, self.stream.cu_stream());
+        }
         unsafe {
             self.stream
                 .launch_builder(&self.expected_q_kernel)
@@ -7961,8 +7965,8 @@ impl GpuDqnTrainer {
                 .arg(&b2)
                 .arg(&b3)
                 .arg(&self.per_sample_support_ptr)
-                .arg(&null_atom_stats)
-                .arg(&null_q_var)
+                .arg(&atom_stats_ptr)
+                .arg(&q_var_ptr)
                 .arg(&atom_positions_buf_ptr)
                 .launch(LaunchConfig {
                     grid_dim: (grid_dim, 1, 1),