dropout: device RNG kernel + Tensor fwd/bwd (T18)

csrc/ops/dropout.cu: counter-based RNG (splitmix64 over seed^index) → fp32
uniform → Bernoulli(keep=1-p); fwd writes out=x⊙mask + an fp32 mask buffer
(per-element 1/(1-p) or 0); bwd applies the same mask (dx=d⊙mask). fp32 + bf16
activation variants (mask fp32 in both; uniform is dtype-independent so masks
match across precisions). Stateless → re-run with same seed = same mask (T13
recompute-safe). Registered in build.rs + FFI decls.

Tensor::dropout(p,seed)->(out,mask) and Tensor::dropout_backward(d,mask) wrap the
launches (contiguous F32/BF16, default stream, per-op sync via the kernels).

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

crates/xtrain-cuda/src/ffi.rs

@@ -447,3 +447,48 @@ 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,
+    );
+}