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

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

.claude/skills/add-jit-kernel/SKILL.md

@@ -27,6 +27,7 @@ Add a new operation that scales each element of a tensor by a scalar factor:
 
 These hold for every step below.
 
+- **`namespace sglang` is where JIT code lives.** Open it after the include block and close it at the end of the file, with the device kernels, traits and host wrapper inside. The shared `host::` / `device::` helpers are nested in it too, so they resolve unqualified. `load_jit` emits the `TVM_FFI_DLL_EXPORT_TYPED_FUNC` wrapper inside `namespace sglang` as well, so the `kernel_name` you pass from Python needs no `sglang::` prefix.
 - **Check where the check is cheapest: `static_assert` > C++ host check > cached Python > per-call Python.** Anything fixed at compile time is a `static_assert`. Anything about the tensors is a `TensorMatcher` / `CHECK_HOST` in the C++ launcher, free next to a kernel launch. A check Python cannot delegate goes inside the `@cache_once` module factory, where it runs once per specialisation. What remains in the per-call entry point costs interpreter time on *every* forward, so it should be nothing but picking the module and allocating `out`.
 - **Fixed-width integer types.** Prefer `int32_t` / `int64_t` / `uint32_t` / `size_t` over `int`, `long`, or `long long`, so an index has the same width on both sides of the FFI boundary. Bare `int` is fine only where the width plainly cannot matter — an unrolled loop counter over a `constexpr` bound, a template `int` parameter. Shapes arrive as `int64_t` (`SymbolicSize::unwrap()`); narrowing to `uint32_t` for in-kernel indexing is a deliberate act, so write the `static_cast` explicitly and only where the range is known.
 - **Doxygen comments in C++.** Document exported entities with `///` or `/** ... */` blocks using `\brief`, `\param`, `\tparam`, `\return`, the way `include/sgl_kernel/` does. `python -m sglang.kernels.jit` writes `CommentFormat: Doxygen` into `.clangd` when clangd is 21 or newer, so these render on hover in the editor. Plain `//` remains fine for implementation notes inside a function body.
@@ -251,7 +252,7 @@ The implementation fully uses the project abstractions described above:
 #include <dlpack/dlpack.h>
 #include <tvm/ffi/container/tensor.h>
 
-namespace {
+namespace sglang {
 
 /**
  * \brief Element-wise scale using vectorized 128-bit loads/stores.
@@ -357,7 +358,7 @@ void scale(tvm::ffi::TensorView dst, tvm::ffi::TensorView src, float factor) {
       n);
 }
 
-}  // namespace
+}  // namespace sglang
 ```
 
 **Key points:**