[muh] add tuning_merge.cuh: BI-V100 tuning header for merge

Translated from CCCL cub/device/dispatch/tuning/tuning_merge.cuh.
Uses hardware_capability dispatch instead of compute_capability.

muh/include/muh/tuning/tuning_merge.cuh

@@ -0,0 +1,55 @@
+// muh/include/muh/tuning/tuning_merge.cuh — BI-V100
+//
+// Mirrors: cccl_upstream/cub/cub/device/dispatch/tuning/tuning_merge.cuh
+// CCCL: threads=256/512, items=nominal_4B scaled, depends on bulk copy support
+//
+// vllm relevance: beam search candidate merging
+// SMEM risk: tile = threads * items * (key_size + value_size), can overflow for large types
+
+#pragma once
+
+#include "muh/hardware.cuh"
+#include "muh/tuning/common.cuh"
+
+namespace muh::tuning::merge {
+
+struct MergePolicy {
+  int threads_per_block;
+  int items_per_thread;
+  CacheLoadModifier load_modifier;
+  BlockStoreAlgorithm store_algorithm;
+  bool use_bulk_copy;
+};
+
+// CCCL's nominal_4B_items_to_items
+constexpr int nominal_4B_items(int nominal, int type_size) {
+  int items = nominal * 4 / type_size;
+  return items < 1 ? 1 : items;
+}
+
+struct policy_selector {
+  int key_size;
+  int value_size;  // 0 for keys-only
+  bool can_bulk_copy;
+
+  constexpr MergePolicy operator()(const hardware_capability& hw) const {
+    int tune_size = key_size + value_size;
+
+    if (hw.at_least(hardware_capability::vendor_t::iluvatar, 100) && can_bulk_copy) {
+      // SM100-like: 512 threads, bulk copy
+      int items = nominal_4B_items(11, tune_size);
+      // SMEM check: 512 * items * tune_size <= 49152
+      while (512 * items * tune_size > hw.max_shared_memory_per_block && items > 1)
+        items--;
+      return {512, items, LOAD_DEFAULT, BLOCK_STORE_WARP_TRANSPOSE, true};
+    }
+
+    // Default: 256 threads, no bulk
+    int items = nominal_4B_items(15, tune_size);
+    while (256 * items * tune_size > hw.max_shared_memory_per_block && items > 1)
+      items--;
+    return {256, items, LOAD_DEFAULT, BLOCK_STORE_WARP_TRANSPOSE, false};
+  }
+};
+
+} // namespace muh::tuning::merge