feat: L3 memory retrieval on the router read path (record-only)

When a user sends input, embed it, query L3 across all sessions, dedup
against in-session turns, and record the retrieval as an event so replay
is deterministic (invariant #9). Hits land in RouterState; context
injection follows in a later slice.

- Add L3MemoryRetrievedEvent + L3RetrievedHit (registered in eventModule)
- RouterState.lastRetrievedMemory + reducer case; RouterTurn carries turnId
- RouterConfig.retrievalK (default 5)
- Harden L3 write path: runCatching + visible logging, cancellation
  re-thrown; event append stays ahead of the L3 write
- Warn prominently when the non-durable in_memory L3 backend is selected

core/events/src/main/kotlin/com/correx/core/events/events/RouterEvents.kt

@@ -19,3 +19,21 @@ enum class ChatTurnRole {
     USER,
     ROUTER,
 }
+
+@Serializable
+@SerialName("L3MemoryRetrieved")
+data class L3MemoryRetrievedEvent(
+    val sessionId: SessionId,
+    val queryTurnId: String,
+    val hits: List<L3RetrievedHit>,
+    val timestampMs: Long,
+) : EventPayload
+
+@Serializable
+data class L3RetrievedHit(
+    val entryId: String,
+    val sourceSessionId: SessionId,
+    val sourceTurnId: String,
+    val text: String,
+    val score: Float,
+)

core/events/src/main/kotlin/com/correx/core/events/serialization/Serialization.kt

@@ -10,6 +10,7 @@ import com.correx.core.events.events.ArtifactValidatingEvent
 import com.correx.core.events.events.ChatSessionStartedEvent
 import com.correx.core.events.events.ChatTurnEvent
 import com.correx.core.events.events.EventPayload
+import com.correx.core.events.events.L3MemoryRetrievedEvent
 import com.correx.core.events.events.InferenceCompletedEvent
 import com.correx.core.events.events.InferenceFailedEvent
 import com.correx.core.events.events.InferenceStartedEvent
@@ -73,6 +74,7 @@ val eventModule = SerializersModule {
         subclass(RiskAssessedEvent::class)
         subclass(ChatSessionStartedEvent::class)
         subclass(ChatTurnEvent::class)
+        subclass(L3MemoryRetrievedEvent::class)
     }
 }