Add LMDB oplog migration path with dual-write cutover support

Introduce LMDB oplog store, migration flags, telemetry/backfill tooling, and parity tests to enable staged Surreal-to-LMDB rollout with rollback coverage.

src/ZB.MOM.WW.CBDDC.Persistence/Lmdb/CBDDCLmdbOplogExtensions.cs

@@ -0,0 +1,54 @@
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+using ZB.MOM.WW.CBDDC.Core.Storage;
+using ZB.MOM.WW.CBDDC.Persistence.Surreal;
+
+namespace ZB.MOM.WW.CBDDC.Persistence.Lmdb;
+
+/// <summary>
+///     Extension methods for adding the LMDB oplog provider and migration feature flags.
+/// </summary>
+public static class CBDDCLmdbOplogExtensions
+{
+    /// <summary>
+    ///     Registers LMDB oplog services and replaces <see cref="IOplogStore" /> with a feature-flag migration router.
+    /// </summary>
+    /// <param name="services">The service collection.</param>
+    /// <param name="optionsFactory">Factory creating LMDB environment options.</param>
+    /// <param name="configureFlags">Optional migration feature-flag configuration.</param>
+    /// <returns>The service collection.</returns>
+    public static IServiceCollection AddCBDDCLmdbOplog(
+        this IServiceCollection services,
+        Func<IServiceProvider, LmdbOplogOptions> optionsFactory,
+        Action<LmdbOplogFeatureFlags>? configureFlags = null)
+    {
+        if (services == null) throw new ArgumentNullException(nameof(services));
+        if (optionsFactory == null) throw new ArgumentNullException(nameof(optionsFactory));
+
+        services.TryAddSingleton(optionsFactory);
+
+        var flags = new LmdbOplogFeatureFlags
+        {
+            UseLmdbOplog = true,
+            DualWriteOplog = true,
+            PreferLmdbReads = false,
+            EnableReadShadowValidation = false
+        };
+        configureFlags?.Invoke(flags);
+        services.TryAddSingleton(flags);
+
+        services.TryAddSingleton<OplogMigrationTelemetry>();
+        services.TryAddSingleton<LmdbOplogStore>();
+
+        bool surrealRegistered = services.Any(descriptor => descriptor.ServiceType == typeof(SurrealOplogStore));
+        if (surrealRegistered)
+        {
+            services.TryAddSingleton<LmdbOplogBackfillTool>();
+            services.Replace(ServiceDescriptor.Singleton<IOplogStore, FeatureFlagOplogStore>());
+        }
+        else
+            services.Replace(ServiceDescriptor.Singleton<IOplogStore>(sp => sp.GetRequiredService<LmdbOplogStore>()));
+
+        return services;
+    }
+}