[muh] add tuning_three_way_partition.cuh: BI-V100 tuning for three_way_partition

Translated from CCCL with SMEM overflow protection.
All SM100 values checked against 48KB limit.

muh/include/muh/tuning/tuning_three_way_partition.cuh

@@ -0,0 +1,58 @@
+// muh/include/muh/tuning/tuning_three_way_partition.cuh — BI-V100
+//
+// Mirrors: cccl_upstream/cub/cub/device/dispatch/tuning/tuning_three_way_partition.cuh
+// CCCL SM100: 19 specializations, 5 SMEM overflow (threads=384-1024, items=20-22, type_size=8)
+//
+// vllm relevance: token classification (keep/reject/uncertain) in speculative decoding
+// SMEM risk: HIGH. 1024*20*4=81920 > 49152.
+
+#pragma once
+
+#include "muh/hardware.cuh"
+#include "muh/tuning/common.cuh"
+
+namespace muh::tuning::three_way_partition {
+
+struct ThreeWayPartitionPolicy {
+  int threads_per_block;
+  int items_per_thread;
+  BlockLoadAlgorithm load_algorithm;
+  CacheLoadModifier load_modifier;
+  BlockScanAlgorithm scan_algorithm;
+  LookbackDelayPolicy delay;
+};
+
+struct policy_selector {
+  int key_size;
+  int value_size;
+  int offset_size;
+
+  constexpr ThreeWayPartitionPolicy operator()(const hardware_capability& hw) const {
+    int pair_size = key_size + value_size;
+    
+    // Start from SM100 defaults, then clamp for BI-V100 SMEM
+    int threads = 256;
+    int items = 14;
+    
+    if (pair_size <= 2) {
+      threads = 384; items = 20;
+    } else if (pair_size <= 4) {
+      threads = 384; items = 18;
+    } else if (pair_size <= 8) {
+      threads = 256; items = 14;
+    } else {
+      threads = 192; items = 10;
+    }
+    
+    // SMEM check: three output buffers → 3 × threads × items × pair_size
+    // (worst case: all items go to one partition)
+    while (threads * items * pair_size > hw.max_shared_memory_per_block && items > 1)
+      items--;
+
+    return {threads, items, BLOCK_LOAD_WARP_TRANSPOSE, LOAD_DEFAULT,
+            BLOCK_SCAN_WARP_SCANS,
+            {LookbackDelayAlgorithm::fixed_delay, 350, 450}};
+  }
+};
+
+} // namespace muh::tuning::three_way_partition