Merge t18-dropout into main

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

# Conflicts:
#	README.md
#	crates/xtrain-autodiff/tests/autograd.rs
#	crates/xtrain-model/src/model.rs
#	crates/xtrain-train/src/bin/train.rs
#	crates/xtrain-train/src/train_loop.rs
#	docs/evolution.md

crates/xtrain-cuda/build.rs

@@ -38,6 +38,7 @@ fn main() {
         .file("../../csrc/ops/attention.cu")
         .file("../../csrc/ops/flash_attention.cu")
         .file("../../csrc/ops/cast.cu")
+        .file("../../csrc/ops/dropout.cu")
         .compile("xtrain_cuda_kernels");
 }