cuda: fix int32 overflow in MoE dense kernels; surface launch errors in release

The dense MoE kernels (moe_replicate, moe_bias_add_3d, moe_weighted_sum)
computed total / expert_stride / element indices in int32. gpt-oss prefill
runs the whole prompt through the dense path unchunked (SPARSE_MAX_TOKENS=8),
so local_experts*num_tokens*hidden (and batch*num_tokens*dim, and
local_id*expert_stride) overflow int32 at ~3.6k-23k prefill tokens
(TP-dependent) — well inside the supported context window. The launch then
fails silently because CUDA_CHECK_LAST_ERROR was ((void)0) under NDEBUG, so
the bias / weighted-sum simply never runs and the forward pass is corrupted
with no error reported.

Fix: switch the three kernels and their launchers to long long, mirroring the
(long long) indexing already used in moe_sparse.cu. Also make
CUDA_CHECK_LAST_ERROR always-on — cudaGetLastError does not sync, so the
per-launch host cost is negligible, and a silent launch failure is exactly
the class of bug this one was.

Verified on dash5 (RTX 5090): a direct kernel test at 2.21B elements (>2^31)
for both moe_replicate and moe_bias_add_3d produces correct results with no
launch error; bench-gpt-oss TP=2 holds at 5.9ms TPOT, output unchanged.

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

csrc/common.cuh

@@ -49,10 +49,12 @@ __device__ __forceinline__ float block_reduce_max(float val) {
     return val;
 }
 
-// --- Launch error checking (debug builds only) ---
-#ifdef NDEBUG
-#define CUDA_CHECK_LAST_ERROR() ((void)0)
-#else
+// --- Launch error checking ---
+// Always on, including release builds. A launch with an invalid config
+// (e.g. 32-bit overflow in grid/index math) is otherwise silent and produces
+// garbage with no clue — the MoE int32-overflow bug was found exactly because
+// release swallowed the launch failure. `cudaGetLastError()` does not
+// synchronize the stream, so the per-launch host cost is negligible.
 #include <cstdio>
 #define CUDA_CHECK_LAST_ERROR() do { \
     cudaError_t err = cudaGetLastError(); \
@@ -61,4 +63,3 @@ __device__ __forceinline__ float block_reduce_max(float val) {
                 __FILE__, __LINE__, cudaGetErrorString(err)); \
     } \
 } while(0)
-#endif