feat(router): capture latency + tokens on ROUTER ChatTurnEvent

ChatTurnEvent gains nullable latencyMs and tokensUsed fields (defaults preserve
backward-compat; legacy JSON without them deserializes cleanly). emitChatTurn
accepts optional metrics; the ROUTER emit site passes inferenceResponse values
through; the USER emit site leaves both null.

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

@@ -1,6 +1,7 @@
 package com.correx.core.events.events
 
 import com.correx.core.events.types.SessionId
+import com.correx.core.inference.TokenUsage
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 
@@ -12,6 +13,8 @@ data class ChatTurnEvent(
     val role: ChatTurnRole,
     val content: String,
     val timestampMs: Long,
+    val latencyMs: Long? = null,
+    val tokensUsed: TokenUsage? = null,
 ) : EventPayload
 
 @Serializable