Implement checkpoint modes with docs/tests and reorganize project file layout

tests/CBDD.Tests.Benchmark/Infrastructure/BenchmarkTransactionHolder.cs

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+using ZB.MOM.WW.CBDD.Core;
+using ZB.MOM.WW.CBDD.Core.Storage;
+using ZB.MOM.WW.CBDD.Core.Transactions;
+
+namespace ZB.MOM.WW.CBDD.Tests.Benchmark;
+
+internal sealed class BenchmarkTransactionHolder : ITransactionHolder, IDisposable
+{
+    private readonly StorageEngine _storage;
+    private readonly object _sync = new();
+    private ITransaction? _currentTransaction;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="BenchmarkTransactionHolder"/> class.
+    /// </summary>
+    /// <param name="storage">The storage engine used to create transactions.</param>
+    public BenchmarkTransactionHolder(StorageEngine storage)
+    {
+        _storage = storage ?? throw new ArgumentNullException(nameof(storage));
+    }
+
+    /// <summary>
+    /// Gets the current active transaction or starts a new one.
+    /// </summary>
+    /// <returns>The current active transaction.</returns>
+    public ITransaction GetCurrentTransactionOrStart()
+    {
+        lock (_sync)
+        {
+            if (_currentTransaction == null || _currentTransaction.State != TransactionState.Active)
+            {
+                _currentTransaction = _storage.BeginTransaction();
+            }
+
+            return _currentTransaction;
+        }
+    }
+
+    /// <summary>
+    /// Gets the current active transaction or starts a new one asynchronously.
+    /// </summary>
+    /// <returns>A task that returns the current active transaction.</returns>
+    public Task<ITransaction> GetCurrentTransactionOrStartAsync()
+    {
+        return Task.FromResult(GetCurrentTransactionOrStart());
+    }
+
+    /// <summary>
+    /// Commits the current transaction when active and clears the holder.
+    /// </summary>
+    public void CommitAndReset()
+    {
+        lock (_sync)
+        {
+            if (_currentTransaction == null)
+            {
+                return;
+            }
+
+            if (_currentTransaction.State == TransactionState.Active ||
+                _currentTransaction.State == TransactionState.Preparing)
+            {
+                _currentTransaction.Commit();
+            }
+
+            _currentTransaction.Dispose();
+            _currentTransaction = null;
+        }
+    }
+
+    /// <summary>
+    /// Rolls back the current transaction when active and clears the holder.
+    /// </summary>
+    public void RollbackAndReset()
+    {
+        lock (_sync)
+        {
+            if (_currentTransaction == null)
+            {
+                return;
+            }
+
+            if (_currentTransaction.State == TransactionState.Active ||
+                _currentTransaction.State == TransactionState.Preparing)
+            {
+                _currentTransaction.Rollback();
+            }
+
+            _currentTransaction.Dispose();
+            _currentTransaction = null;
+        }
+    }
+
+    /// <summary>
+    /// Disposes this holder and rolls back any outstanding transaction.
+    /// </summary>
+    public void Dispose()
+    {
+        RollbackAndReset();
+    }
+}