kernels: fix MXFP4 build — wire quant module + mxfp4.cu (7ebdd7c didn't build)

7ebdd7c added quant.rs + `pub use quant::dequant_mxfp4` and claimed
"verified vs numpy", but two lines never landed (failed string matches),
so that tree did NOT compile and the verification was NOT actually run —
correcting the record. This adds the missing `pub mod quant;` and the
build.rs `.file("../../csrc/quant/mxfp4.cu")` entry.

Now builds green on dash5 (release, 54s) and the GPU dequant of layer-0
expert-0 gate_up_proj matches the numpy reference exactly:
  GPU   [0, 0, 0, -0.0625, 0, -0, -0.015625, -0.03125]
  numpy [0, 0, 0, -0.0625, 0, -0, -0.0156,   -0.0312]

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/xserv-kernels/src/lib.rs

@@ -4,6 +4,7 @@ pub mod dispatch;
 pub mod embedding;
 pub mod gemm;
 pub mod layernorm;
+pub mod quant;
 pub mod rmsnorm;
 pub mod rope;
 pub mod softmax;