Add thread-safe RiaStringPool utility and unit tests

Introduce RiaStringPool for efficient shared string storage and retrieval by index, with thread-safe access using shared mutex. Pre-allocate empty string at index 0. Add comprehensive unit tests covering core functionality, concurrency, edge cases, and realistic usage.

ApplicationLibCode/Application/RiaStringPool.h

@@ -0,0 +1,57 @@
+/////////////////////////////////////////////////////////////////////////////////
+//
+//  Copyright (C) 2026     Equinor ASA
+//
+//  ResInsight is free software: you can redistribute it and/or modify
+//  it under the terms of the GNU General Public License as published by
+//  the Free Software Foundation, either version 3 of the License, or
+//  (at your option) any later version.
+//
+//  ResInsight is distributed in the hope that it will be useful, but WITHOUT ANY
+//  WARRANTY; without even the implied warranty of MERCHANTABILITY or
+//  FITNESS FOR A PARTICULAR PURPOSE.
+//
+//  See the GNU General Public License at <http://www.gnu.org/licenses/gpl.html>
+//  for more details.
+//
+/////////////////////////////////////////////////////////////////////////////////
+
+#pragma once
+
+#include <cstdint>
+#include <shared_mutex>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+//==================================================================================================
+/// String pool for memory-efficient storage of shared strings
+/// Stores strings once and returns indices for lookups
+//==================================================================================================
+class RiaStringPool
+{
+public:
+    using IndexType                          = uint32_t;
+    static constexpr IndexType INVALID_INDEX = static_cast<IndexType>( -1 );
+
+    static RiaStringPool& instance();
+
+    // Get or create an index for a string
+    IndexType getIndex( const std::string& str );
+
+    // Get string by index
+    const std::string& getString( IndexType index ) const;
+
+    // Get empty string index
+    IndexType getEmptyIndex() const { return m_emptyIndex; }
+
+private:
+    RiaStringPool();
+    RiaStringPool( const RiaStringPool& )            = delete;
+    RiaStringPool& operator=( const RiaStringPool& ) = delete;
+
+    mutable std::shared_mutex                  m_mutex;
+    std::vector<std::string>                   m_strings;
+    std::unordered_map<std::string, IndexType> m_stringToIndex;
+    IndexType                                  m_emptyIndex;
+};