[muh] add tuning_segmented_scan.cuh: BI-V100 tuning header for segmented_scan

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

muh/include/muh/tuning/tuning_segmented_scan.cuh

@@ -0,0 +1,45 @@
+// muh/include/muh/tuning/tuning_segmented_scan.cuh — BI-V100
+//
+// Mirrors: cccl_upstream/cub/cub/device/dispatch/tuning/tuning_segmented_scan.cuh
+// CCCL: threads=128, items=9 (nominal), scale_mem_bound for SMEM
+//
+// vllm relevance: per-sequence softmax denominator accumulation
+// SMEM risk: scale_mem_bound handles it. tuple<AccumT, bool> → effective size = accum_size + 1 (padded to alignment)
+
+#pragma once
+
+#include "muh/hardware.cuh"
+#include "muh/tuning/common.cuh"
+
+namespace muh::tuning::segmented_scan {
+
+struct SegmentedScanPolicy {
+  int threads_per_block;
+  int items_per_thread;
+  int max_segments_per_block;
+  BlockLoadAlgorithm load_algorithm;
+  BlockStoreAlgorithm store_algorithm;
+  BlockScanAlgorithm scan_algorithm;
+};
+
+struct policy_selector {
+  int accum_size;
+  int accum_align;
+
+  constexpr SegmentedScanPolicy operator()(const hardware_capability& /*hw*/) const {
+    constexpr int nominal_threads = 128;
+    constexpr int nominal_items = 9;
+    constexpr int max_segments = 512;
+
+    // Multi-segment agent uses tuple<AccumT, bool>: size = round_up(accum_size + 1, accum_align)
+    int tuple_size = accum_size + accum_align;  // conservative estimate
+    auto [items, threads] = scale_mem_bound(nominal_threads, nominal_items, tuple_size);
+
+    return {threads, items, max_segments,
+            BLOCK_LOAD_WARP_TRANSPOSE,
+            BLOCK_STORE_WARP_TRANSPOSE,
+            BLOCK_SCAN_WARP_SCANS};
+  }
+};
+
+} // namespace muh::tuning::segmented_scan