feat: event-source router CHAT and STEERING conversation turns

Adds ChatTurnEvent (role USER|ROUTER) so router conversation history
survives restart and is rebuilt from the event log via projection,
restoring Hard Invariant #1 for the CHAT path. Both CHAT and STEERING
now emit user + router turn events; STEERING additionally keeps its
existing SteeringNoteAddedEvent so the structured directive record is
preserved alongside the conversation turns.

RouterFacade drops the in-memory ConcurrentHashMap of histories and
rebuilds state through the repository after each emission. Reducer
appends turns to conversationHistory. ChatTurnEvent is registered in
eventModule.

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

@@ -0,0 +1,21 @@
+package com.correx.core.events.events
+
+import com.correx.core.events.types.SessionId
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+@Serializable
+@SerialName("ChatTurn")
+data class ChatTurnEvent(
+    val sessionId: SessionId,
+    val turnId: String,
+    val role: ChatTurnRole,
+    val content: String,
+    val timestampMs: Long,
+) : EventPayload
+
+@Serializable
+enum class ChatTurnRole {
+    USER,
+    ROUTER,
+}

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

@@ -8,6 +8,7 @@ import com.correx.core.events.events.ArtifactCreatedEvent
 import com.correx.core.events.events.ArtifactValidatedEvent
 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.InferenceCompletedEvent
 import com.correx.core.events.events.InferenceFailedEvent
@@ -71,6 +72,7 @@ val eventModule = SerializersModule {
         subclass(RetryAttemptedEvent::class)
         subclass(RiskAssessedEvent::class)
         subclass(ChatSessionStartedEvent::class)
+        subclass(ChatTurnEvent::class)
     }
 }