fix(ml-alpha): drop synchronize() in per-horizon pool + head bindings

After adaf275af removed the synchronizes in PerHorizonTrainState's
forward_with_blend / backward_through_blend, smoke alpha-perception-9l6hw
still failed with CUDA_ERROR_STREAM_CAPTURE_UNSUPPORTED. Root cause:
PerHorizonAttentionPool::{forward,backward} and PerHorizonResidualHead::
{forward,backward} each end with their own self.stream.synchronize(),
which is illegal during CUDA Graph capture.

Same fix: drop the four synchronizes. Same-stream issue order ensures
the next kernel sees the previous one's output. Capture invariant
preserved.

Verified locally: per_horizon_full_pipeline_smoke (2/2), attention pool
numgrad (1/1), residual head numgrad (1/1) all pass on RTX 3050.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/ml-alpha/src/per_horizon_attention_pool.rs

@@ -82,7 +82,8 @@ impl PerHorizonAttentionPool {
                 .launch(cfg)
                 .context("per_horizon_attention_pool_fwd")?;
         }
-        self.stream.synchronize()?;
+        // No synchronize: same-stream issue order is sufficient and
+        // synchronize is illegal during CUDA Graph capture.
         Ok(())
     }
 
@@ -122,7 +123,8 @@ impl PerHorizonAttentionPool {
                 .launch(cfg)
                 .context("per_horizon_attention_pool_bwd")?;
         }
-        self.stream.synchronize()?;
+        // No synchronize: same-stream issue order is sufficient and
+        // synchronize is illegal during CUDA Graph capture.
         Ok(())
     }
 }

crates/ml-alpha/src/per_horizon_residual_head.rs

@@ -71,7 +71,8 @@ impl PerHorizonResidualHead {
                 .launch(cfg)
                 .context("per_horizon_residual_head_fwd")?;
         }
-        self.stream.synchronize()?;
+        // No synchronize: same-stream issue order is sufficient and
+        // synchronize is illegal during CUDA Graph capture.
         Ok(())
     }
 
@@ -107,7 +108,8 @@ impl PerHorizonResidualHead {
                 .launch(cfg)
                 .context("per_horizon_residual_head_bwd")?;
         }
-        self.stream.synchronize()?;
+        // No synchronize: same-stream issue order is sufficient and
+        // synchronize is illegal during CUDA Graph capture.
         Ok(())
     }
 }