Root cause: `SMOKE_CUDA: OnceLock<MlDevice>` held a static Arc<CudaContext>
for the process lifetime. CudaSlice Drop from test N recorded errors on
this shared context's error_state, causing test N+1's bind_to_thread() to
fail with CUDA_ERROR_INVALID_VALUE.
Fix: create a fresh MlDevice per test (no static caching). Each test gets
its own CudaContext Arc with clean error_state.
Also: convert all GPU smoke tests from #[tokio::test] to synchronous #[test]
with explicit tokio::runtime::Builder::new_current_thread(). The runtime is
explicitly dropped between tests, ensuring all Arc<CudaContext> refs are freed.
Result: 5 of 6 sequential smoke tests now pass. The remaining 1 failure is
a real Candle GpuTensor bug: the replay buffer insertion path still uses
Candle's elementwise kernels, which cache CudaFunction handles that become
stale across test boundaries. Fix: eliminate Candle from replay buffer path.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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