fix: resolve all test warnings — wrap orphaned tests, drop unused bindings

- gpu_replay_buffer.rs: orphaned test functions (test_creation, test_beta,
  test_clear) and make_stream helper were at module level without #[cfg(test)]
  mod tests wrapper. Added proper module boundary.

- gpu_residency.rs, training_stability.rs: sample_proportional() called for
  side effect (populates internal buffers asserted on next line). Dropped
  unused batch/batch2 bindings.

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

crates/ml-dqn/src/gpu_replay_buffer.rs

@@ -918,7 +918,8 @@ fn a32i(s: &Arc<CudaStream>, n: usize, nm: &str) -> Result<CudaSlice<i32>, MLErr
     s.alloc_zeros::<i32>(n).map_err(|e| MLError::ModelError(format!("alloc {nm}: {e}")))
 }
 
-/// DtoD clone of first `n` elements from `src` into a new allocation (async on stream).
+#[cfg(test)]
+mod tests {
     use super::*;
 
     fn make_stream() -> Arc<CudaStream> {
@@ -951,3 +952,4 @@ fn a32i(s: &Arc<CudaStream>, n: usize, nm: &str) -> Result<CudaSlice<i32>, MLErr
         b.step(); b.clear().expect("clear");
         assert_eq!(b.len(), 0); assert_eq!(b.current_step, 0);
     }
+}

crates/ml/src/trainers/dqn/smoke_tests/gpu_residency.rs

@@ -193,7 +193,7 @@ fn test_gpu_replay_buffer_priority_update_valid() -> anyhow::Result<()> {
     let idx = batch.indices_ptr; buf.update_priorities_gpu_raw(idx, &td_errors, 16)?;
 
     // Verify sampling still works with updated priorities (proves update succeeded)
-    let batch2 = buf.sample_proportional(16)?;
+    buf.sample_proportional(16)?;
     let weights_host = dtoh_f32(buf.sample_weights_ref(), &stream)?;
 
     // Weights should still be positive after priority update
@@ -221,7 +221,7 @@ fn test_searchsorted_gpu_correctness() -> anyhow::Result<()> {
 
     // Sample multiple times to exercise the searchsorted codepath (GPU-resident)
     for trial in 0..5 {
-        let batch = buf.sample_proportional(10)?;
+        buf.sample_proportional(10)?;
         let indices_host = dtoh_u32(buf.sample_indices_ref(), &stream)?;
         for (i, &idx) in indices_host.iter().enumerate() {
             assert!(

crates/ml/src/trainers/dqn/smoke_tests/training_stability.rs

@@ -114,7 +114,7 @@ fn test_per_weights_valid() -> anyhow::Result<()> {
         buf.insert_batch(&states, &next_states, &actions, &rewards, &dones, 1)?;
     }
 
-    let batch = buf.sample_proportional(16)?;
+    buf.sample_proportional(16)?;
 
     // Download f32 weights to host for assertion
     let mut weights_host = vec![0.0_f32; 16];
@@ -158,7 +158,7 @@ fn test_per_indices_valid() -> anyhow::Result<()> {
         buf.insert_batch(&states, &next_states, &actions, &rewards, &dones, 1)?;
     }
 
-    let batch = buf.sample_proportional(16)?;
+    buf.sample_proportional(16)?;
 
     // Download indices to host for assertion
     let mut indices_host = vec![0_u32; 16];