feat(sessions): add bounded replay ring buffer to SessionEventDistributor

src/ZB.MOM.WW.MxGateway.Server/Configuration/EventOptions.cs

@@ -11,4 +11,20 @@ public sealed class EventOptions
     /// Gets the backpressure policy for event queue overflow.
     /// </summary>
     public EventBackpressurePolicy BackpressurePolicy { get; init; } = EventBackpressurePolicy.FailFast;
+
+    /// <summary>
+    ///     Gets the maximum number of events retained in the per-session replay ring buffer
+    ///     used to re-deliver events a returning subscriber missed (reconnect/reattach).
+    ///     When the buffer exceeds this count the oldest retained events are evicted first.
+    ///     A value of <c>0</c> disables replay retention entirely.
+    /// </summary>
+    public int ReplayBufferCapacity { get; init; } = 1024;
+
+    /// <summary>
+    ///     Gets the maximum age, in seconds, of an event retained in the per-session replay
+    ///     ring buffer. Entries older than this are evicted regardless of capacity. A value
+    ///     of <c>0</c> disables age-based eviction (only <see cref="ReplayBufferCapacity"/>
+    ///     bounds the buffer).
+    /// </summary>
+    public double ReplayRetentionSeconds { get; init; } = 300;
 }

src/ZB.MOM.WW.MxGateway.Server/Configuration/GatewayOptionsValidator.cs

@@ -193,6 +193,16 @@ public sealed class GatewayOptionsValidator : OptionsValidatorBase<GatewayOption
         {
             builder.Add("MxGateway:Events:BackpressurePolicy must be a supported backpressure policy.");
         }
+
+        // ReplayBufferCapacity and ReplayRetentionSeconds are bounds on the replay ring
+        // buffer; 0 is a valid value (disables that dimension), so only negatives fail.
+        AddIfNegative(
+            options.ReplayBufferCapacity,
+            "MxGateway:Events:ReplayBufferCapacity must be greater than or equal to zero.",
+            builder);
+        builder.RequireThat(
+            options.ReplayRetentionSeconds >= 0,
+            "MxGateway:Events:ReplayRetentionSeconds must be greater than or equal to zero.");
     }
 
     private static void ValidateDashboard(DashboardOptions options, ValidationBuilder builder)