[jit_kernel] Move JIT kernels into namespace sglang (#33400)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

docs/docs/developer_guide/development_jit_kernel_guide.mdx

@@ -23,6 +23,11 @@ After generating the file, restart the clangd language server. It should now rec
 C++ source code is located in `python/sglang/kernels/jit/csrc`.
 Reusable functions should be placed in `python/sglang/kernels/jit/include`.
 
+JIT C++ lives in `namespace sglang`: open it after the include block and close it at the
+end of the file, with the device kernels and the host wrapper both inside.
+The shared `host::` and `device::` helpers are nested in it as well, so they resolve unqualified
+and need no `sglang::` prefix.
+
 We use [tvm-ffi](https://github.com/apache/tvm-ffi) for efficient foreign language bindings.
 Refer to the [documentation](https://tvm.apache.org/ffi/) for advanced usage, such as exporting C++ objects.
 Typically, `tvm::ffi::TensorView` is sufficient for passing PyTorch Tensors from Python.
@@ -33,6 +38,8 @@ Python interfaces are defined in `python/sglang/kernels/jit`.
 The `load_jit` utility function in `python/sglang/kernels/jit/utils/compile.py` loads and returns the compiled module.
 To export a C++ function (e.g., `cpp_func`), pass `cuda_wrappers=[("func", "cpp_func")]` to `load_jit`.
 The function can then be called in Python as `module.func`.
+`load_jit` emits the export wrapper inside `namespace sglang`, so write `cpp_func` without a
+`sglang::` prefix.
 
 For caching compiled modules, prefer `sglang.kernels.jit.utils.cache_once` over `functools.lru_cache`.
 `functools.lru_cache` is not compatible with `torch.compile`.
@@ -174,7 +181,7 @@ Write your CUDA kernel in [kernels/jit/csrc/add_constant.cuh](https://github.com
 #include <cstddef>
 #include <cstdint>
 
-namespace {
+namespace sglang {
 
 template <int32_t kConstant>
 __global__ void add_constant_kernel(int32_t* dst, const int32_t* src, size_t length) {
@@ -217,7 +224,7 @@ void add_constant(tvm::ffi::TensorView dst, tvm::ffi::TensorView src) {
       num_elements);
 }
 
-}  // namespace
+}  // namespace sglang
 
 ```