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/src/ffi.rs

@@ -500,3 +500,48 @@ unsafe extern "C" {
         s: CudaStream,
     );
 }
+
+// Dropout (Phase T18, csrc/ops/dropout.cu). A counter-based (stateless) RNG: the
+// keep/drop decision for element `i` is `hash(seed, i)` — no global state, so a
+// re-run with the same `seed` reproduces the same mask (compatible with T13
+// activation recomputation). Forward writes `out = x ⊙ mask` and the fp32 `mask`
+// buffer (mask[i] = (1/(1-p)) if kept else 0, the inverted-dropout scale);
+// backward applies the SAME mask: dx = d ⊙ mask. fp32 + bf16 activation variants
+// (mask is fp32 in both; the uniform is computed in fp32, dtype-independent).
+#[cfg(not(no_cuda))]
+unsafe extern "C" {
+    pub fn launch_dropout_fwd_f32(
+        x: *const f32,
+        out: *mut f32,
+        mask: *mut f32,
+        p: f32,
+        scale: f32,
+        seed: u64,
+        n: i32,
+        s: CudaStream,
+    );
+    pub fn launch_dropout_bwd_f32(
+        d: *const f32,
+        mask: *const f32,
+        dx: *mut f32,
+        n: i32,
+        s: CudaStream,
+    );
+    pub fn launch_dropout_fwd_bf16(
+        x: *const c_void,
+        out: *mut c_void,
+        mask: *mut f32,
+        p: f32,
+        scale: f32,
+        seed: u64,
+        n: i32,
+        s: CudaStream,
+    );
+    pub fn launch_dropout_bwd_bf16(
+        d: *const c_void,
+        mask: *const f32,
+        dx: *mut c_void,
+        n: i32,
+        s: CudaStream,
+    );
+}