perf: streams / drop per-op sync

Default-stream kernels run in order and every host read goes through a
stream-ordered cudaMemcpy (to_device), so the per-op cudaDeviceSynchronize
after each kernel was pure overhead — remove all 21 in tensor.rs. Host
data is still correctly ordered by the D2H memcpy that reads it.

Also zero op-output buffers with cudaMemset (device-side, async) instead of
a blocking H2D memcpy of a host zero buffer on every allocation — that
copy was itself a hidden per-op sync point.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/xtrain-tensor/src/storage.rs

@@ -99,9 +99,11 @@ impl Storage {
         match device {
             Device::Cpu => Ok(Storage::cpu(vec![0u8; len_bytes])),
             Device::Cuda(dev) => {
-                // No device memset in T2: stage zeros from the host.
+                // Device-side memset (Phase T7): avoids a blocking H2D memcpy of a
+                // host zero buffer on every op-output allocation. cudaMemset is
+                // async on the default stream, so it doesn't serialize the stream.
                 let mut buf = GpuBuffer::alloc(len_bytes)?;
-                buf.copy_from_host(&vec![0u8; len_bytes])?;
+                buf.memset(0)?;
                 Ok(Storage::cuda(buf, dev))
             }
         }