review cleanups: pp+gpt-oss guard, sparse GEMV asserts, warnings

- --pp with gpt-oss now fails with a clear message instead of a
  cryptic missing-weight panic inside the Qwen3-only PP engine.
- Sparse GEMV wrappers assert K%16==0 (FP8) / K%32==0 (MXFP4) — the
  uint4-vectorized kernels would silently drop a tail otherwise.
- Document the topk_ids buffer holding i32 under an F32 dtype label
  (DType has no I32).
- Drop unused imports/locals and the cuBLASLt scale-mode constants
  orphaned by the strided-batched FP8 rework (e631a71).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/xserv-kernels/src/embedding.rs

@@ -1,5 +1,4 @@
 use std::ffi::c_void;
-use xserv_cuda::GpuBuffer;
 use xserv_tensor::{DType, Device, Tensor};
 
 unsafe extern "C" {