Fix B2 indexer: increase TMEM_COLS to 512 for full 128-row MMA output

The MMA produces 128 rows × 128 cols = 4 row-groups × 128 TMEM cols = 512 total.
Even though we only read rows 0-63, the MMA writes all 128 rows.
TMEM_COLS must match the MMA output size, not just the read size.

dsv4/kernels/cuda/indexer_fp8_score_topk.cu

@@ -149,8 +149,7 @@ indexer_fp8_score_topk_kernel(
     constexpr int MMA_K_F8 = 32;
     constexpr int NKT = 4;               // ihd=128 / MMA_K_F8=32
     constexpr int TILE_F8 = 128 * 32;    // 4096 bytes per SMEM tile
-    constexpr int TMEM_COLS = 256;  // 128 rows × 128 cols needs 4×128 = 512,
-                                         // but only 64 rows used (2×128 = 256)
+    constexpr int TMEM_COLS = 512;  // 128 rows × 128 cols → 4 row-groups × 128 cols = 512
 
     const int tid = threadIdx.x;
     const int wid = tid >> 5;