ref(upstream): FULL TREE — Deep-Spark xllm (1470) + ds_vllm csrc/models (703)

Replaces cherry-picked upstream_ref with complete source trees.

xllm/ — Iluvatar official C++ inference engine (15MB, 1470 files)
  Complete: kernels → layers → models → runtime → scheduler → api
  Excluded: .git, binary images, third_party submodule checkouts

ds_vllm/ — Iluvatar official vllm fork (8MB, 703 files)
  Included: csrc/ (ALL CUDA kernels), fused_moe/, qwen3_5 model, _custom_ops
  Excluded: tests, benchmarks, docs, examples (not needed for reference)

Critical call chains now fully traceable:
  MoE: moe_topk_softmax_kernels.cuh → ixformer.h → fused_moe.cpp → layer
  GDN: qwen3_gated_delta_net_base.cpp → qwen3_5_gated_delta_net.cpp
  Attention: ixformer.h → xllm_paged_attention → attention.cpp

upstream_ref/xllm/.github/CONTRIBUTING.md

@@ -0,0 +1,45 @@
+<!-- Copyright 2025 JD.com
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this project except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License. -->
+
+[English](./CONTRIBUTING.md) | [中文](./CONTRIBUTING_zh.md)
+
+# Contribute to xLLM
+
++ Write / translate / fix our documentation
++ Raise questions / Answer questions
++ Provide demos, examples or test cases
++ Give suggestions or other comments
++ Paticipate in [issues](https://github.com/xxx/xLLM/issues) or [discussions](https://github.com/xxx/xLLM/discussions)
++ Pull requests
++ Sharing related research / application
++ Any other ways to improve xLLM
+
+For developers who want to contribute to our code, here is the guidance:
+
+## 1. Choose an issue to contribute
++ Issues with label `PR welcome`, which means:
+    + A reproducible bug
+    + A function in plan
+
+## 2. Install environment for development
++ We strongly suggest you to read our **[Document](http://xxx/docs/)** before developing
++ For setting environment, please check our  **[Readme file](/README.md)**
+
+## 3. Build our project
++ You could run our demo to check whether the requirements are successfully installed:
+
+## 4. Test
+
+After the PR is submitted, we will format and test the code.
+Our tests are still far from perfect, so you are welcomed to add tests to our project!