fix: restore scoped event tracking disable during CUDA Graph capture

The previous commit removed disable_event_tracking() from graph capture,
causing CUDA_ERROR_STREAM_CAPTURE_INVALIDATED on H100 (events recorded
during capture invalidate the capture).

Fix: disable event tracking BEFORE begin_capture, re-enable AFTER end_capture.
With GPU-native cat/stack (no to_host roundtrip), the re-enable is now safe —
post-capture operations don't call to_host on graph-modified buffers.

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

crates/ml/src/cuda_pipeline/gpu_dqn_trainer.rs

@@ -1183,8 +1183,9 @@ impl GpuDqnTrainer {
         self.stream.synchronize()
             .map_err(|e| MLError::ModelError(format!("stream sync before capture: {e}")))?;
 
-        // Event tracking is already disabled at DQNTrainer construction (single-stream
-        // context — events are unnecessary with explicit sync barriers).
+        // Disable event tracking during capture — cudarc's device_ptr/device_ptr_mut
+        // record CudaEvents which are DISALLOWED inside CUDA Graph capture.
+        unsafe { self.stream.context().disable_event_tracking(); }
 
         // Begin stream capture — only work submitted from this thread on this
         // stream is captured (THREAD_LOCAL mode, safe for single-stream use).
@@ -1205,10 +1206,10 @@ impl GpuDqnTrainer {
             cudarc::driver::sys::CUgraphInstantiate_flags::CUDA_GRAPH_INSTANTIATE_FLAG_AUTO_FREE_ON_LAUNCH,
         );
 
-        // Keep event tracking disabled — graph capture wrote stale events to
-        // CudaSlices. All subsequent operations use explicit stream sync +
-        // ManuallyDrop guards (raw_device_ptr), so event tracking is unnecessary.
-        // Re-enabling would leave stale events that cause CUDA_ERROR_INVALID_VALUE.
+        // Re-enable event tracking after capture. With GPU-native cat/stack
+        // (no to_host roundtrip), stale events from capture don't cause issues
+        // because all post-capture GPU ops use DtoD copies, not DtoH.
+        unsafe { self.stream.context().enable_event_tracking(); }
 
         // Propagate submission error first
         submit_result?;