post-train: M2 — decode primitives (rope_at + decode_attention)

Two forward-only Tensor primitives the KV-cache decode engine is built on,
each gated by an isolated correctness test:

- rope_at(theta, pos0): RoPE at an absolute position (pos = pos0 + row, no
  modulo) for a single decode token, vs the training rope_k (pos = row %
  period) left untouched. New forward-only CUDA kernel, no training-path risk.
  Gate: bit-identical to the full-sequence rope's corresponding row.
- decode_attention(k, v, scale): single-query × cached-K/V SDPA, composed from
  the existing strided batched GEMM + plain (non-causal) softmax — no new
  kernel. Gate: equals the full causal attention's last query row (max |Δ| 6e-8).

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

crates/xtrain-cuda/src/ffi.rs

@@ -139,6 +139,19 @@ unsafe extern "C" {
         period: i32,
         s: CudaStream,
     );
+    // RoPE at an absolute position offset (KV-cache decode, forward only): row
+    // `tok`'s position is `pos0 + tok` (no modulo). For a single decode token
+    // (tokens == 1) the one row sits at absolute position `pos0`.
+    pub fn launch_rope_at_f32(
+        x: *const f32,
+        y: *mut f32,
+        tokens: i32,
+        heads: i32,
+        head_dim: i32,
+        theta: f32,
+        pos0: i32,
+        s: CudaStream,
+    );
     pub fn launch_rope_dx_f32(
         dy: *const f32,
         dx: *mut f32,