Optimize hashtable for better cache locality and load factor

- Add cache-aligned entry structures (alignas(16))
- Improve load factor from 75% to 62.5% for better performance
- Add memory prefetching hints for cache optimization
- Implement Robin Hood probing distance tracking
- Add comprehensive performance monitoring metrics
- Include performance test harness for benchmarking

These changes target the Round 1 performance improvements outlined
in the Z3 performance plan, focusing on reducing cache misses and
improving memory access patterns in hash table operations.

hashtable_perf_test.cpp

@@ -0,0 +1,69 @@
+#include "src/util/hashtable.h"
+#include "src/util/hash.h"
+#include <iostream>
+#include <chrono>
+#include <vector>
+#include <random>
+
+struct int_hash {
+    unsigned operator()(int k) const { return static_cast<unsigned>(k); }
+};
+
+struct int_eq {
+    bool operator()(int a, int b) const { return a == b; }
+};
+
+typedef hashtable<int, int_hash, int_eq> int_hashtable;
+
+void test_hashtable_performance() {
+    std::cout << "Testing hashtable performance optimization..." << std::endl;
+
+    const int num_operations = 100000;
+    int_hashtable table;
+
+    std::random_device rd;
+    std::mt19937 gen(rd());
+    std::uniform_int_distribution<> dis(1, 1000000);
+
+    std::vector<int> test_data;
+    for (int i = 0; i < num_operations; ++i) {
+        test_data.push_back(dis(gen));
+    }
+
+    // Test insertion performance
+    auto start = std::chrono::high_resolution_clock::now();
+
+    for (int i = 0; i < num_operations; ++i) {
+        table.insert(test_data[i]);
+    }
+
+    auto end = std::chrono::high_resolution_clock::now();
+    auto duration = std::chrono::duration_cast<std::chrono::microseconds>(end - start);
+
+    std::cout << "Inserted " << num_operations << " elements in " << duration.count() << " microseconds" << std::endl;
+    std::cout << "Table size: " << table.size() << std::endl;
+    std::cout << "Table capacity: " << table.capacity() << std::endl;
+    std::cout << "Load factor: " << table.get_load_factor() << std::endl;
+    std::cout << "Effective load factor: " << table.get_effective_load_factor() << std::endl;
+
+    // Test lookup performance
+    start = std::chrono::high_resolution_clock::now();
+
+    int found = 0;
+    for (int i = 0; i < num_operations; ++i) {
+        if (table.contains(test_data[i])) {
+            found++;
+        }
+    }
+
+    end = std::chrono::high_resolution_clock::now();
+    duration = std::chrono::duration_cast<std::chrono::microseconds>(end - start);
+
+    std::cout << "Looked up " << num_operations << " elements (" << found << " found) in " << duration.count() << " microseconds" << std::endl;
+    std::cout << "Performance test completed." << std::endl;
+}
+
+int main() {
+    test_hashtable_performance();
+    return 0;
+}