Avoid unnecessary allocations in AccumulatorStack

replacing the vector with an array resulted in a .45+-.10 % speedup.

closes https://github.com/official-stockfish/Stockfish/pull/6352

no functional change

src/nnue/network.h

@@ -106,8 +106,6 @@ class Network {
 
     template<IndexType Size>
     friend struct AccumulatorCaches::Cache;
-
-    friend class AccumulatorStack;
 };
 
 // Definitions of the network types

src/nnue/nnue_accumulator.h

@@ -25,7 +25,6 @@
 #include <cstddef>
 #include <cstdint>
 #include <cstring>
-#include <vector>
 
 #include "../types.h"
 #include "nnue_architecture.h"
@@ -48,7 +47,7 @@ template<IndexType Size>
 struct alignas(CacheLineSize) Accumulator {
     std::int16_t               accumulation[COLOR_NB][Size];
     std::int32_t               psqtAccumulation[COLOR_NB][PSQTBuckets];
-    std::array<bool, COLOR_NB> computed;
+    std::array<bool, COLOR_NB> computed = {};
 };
 
 
@@ -142,10 +141,6 @@ struct AccumulatorState {
 
 class AccumulatorStack {
    public:
-    AccumulatorStack() :
-        accumulators(MAX_PLY + 1),
-        size{1} {}
-
     [[nodiscard]] const AccumulatorState& latest() const noexcept;
 
     void reset() noexcept;
@@ -178,8 +173,8 @@ class AccumulatorStack {
                                      const FeatureTransformer<Dimensions>& featureTransformer,
                                      const std::size_t                     end) noexcept;
 
-    std::vector<AccumulatorState> accumulators;
-    std::size_t                   size;
+    std::array<AccumulatorState, MAX_PLY + 1> accumulators;
+    std::size_t                               size = 1;
 };
 
 }  // namespace Stockfish::Eval::NNUE