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

docs/GatewayConfiguration.md

@@ -41,7 +41,9 @@ paths, timeouts, queue sizes, enum values, or protocol values are invalid.
     },
     "Events": {
       "QueueCapacity": 10000,
-      "BackpressurePolicy": "FailFast"
+      "BackpressurePolicy": "FailFast",
+      "ReplayBufferCapacity": 1024,
+      "ReplayRetentionSeconds": 300
     },
     "Dashboard": {
       "Enabled": true,
@@ -135,12 +137,19 @@ ordering and avoids competing consumers.
 |--------|---------|-------------|
 | `MxGateway:Events:QueueCapacity` | `10000` | Capacity for bounded per-session event queues used by the gateway worker event channel and the public gRPC event stream queue. |
 | `MxGateway:Events:BackpressurePolicy` | `FailFast` | Event backpressure behavior. `FailFast` faults the session on public stream queue overflow. `DisconnectSubscriber` disconnects only the slow stream. |
+| `MxGateway:Events:ReplayBufferCapacity` | `1024` | Maximum number of events retained per session in the replay ring buffer, used to re-deliver events a returning subscriber missed (reconnect/reattach). The oldest retained event is evicted once this count is exceeded. `0` disables replay retention. |
+| `MxGateway:Events:ReplayRetentionSeconds` | `300` | Maximum age, in seconds, of an event retained in the replay ring buffer. Entries older than this are evicted regardless of capacity. `0` disables age-based eviction. |
 
 `QueueCapacity` must be greater than zero. With `FailFast`, queue overflow
 faults the affected worker or session instead of silently dropping MXAccess
 events. With `DisconnectSubscriber`, public gRPC stream overflow terminates only
 the affected stream while the MXAccess session remains active.
 
+`ReplayBufferCapacity` and `ReplayRetentionSeconds` must each be greater than or
+equal to zero (either dimension can be disabled with `0`). A returning subscriber
+that asks for events older than the oldest still-retained event is told it missed
+events (a "gap") and must re-snapshot; whatever is still retained is replayed.
+
 ## Dashboard Options
 
 | Option | Default | Description |