Zero-copy intranode for EPv2

EPv1's `low_latency_combine` supports `zero_copy` ([#79](https://github.com/deepseek-ai/DeepEP/pull/79)), but EPv2's `ElasticBuffer` does not — `recv_x` is always freshly allocated and a full-SM copy epilogue always runs:

https://github.com/deepseek-ai/DeepEP/blob/b306af06afd412c88e51e71802951606e40b7358/csrc/elastic/buffer.hpp#L1013

For intranode NVLink-only workloads, this epilogue takes 10% of total time. Would it be possible to support a zero-copy mode for EPv2's intranode path — either by returning a view into the symmetric buffer directly, or by fusing the scatter into the dispatch kernel?