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/router/src/main/kotlin/com/correx/core/router/RouterFacade.kt

@@ -18,6 +18,7 @@ import com.correx.core.inference.InferenceRequest
 import com.correx.core.inference.InferenceRouter
 import com.correx.core.inference.ModelCapability
 import com.correx.core.inference.ResponseFormat
+import com.correx.core.inference.TokenUsage
 import com.correx.core.router.l3.L3MemoryEntry
 import com.correx.core.router.l3.L3MemoryStore
 import com.correx.core.router.l3.L3Query
@@ -148,7 +149,11 @@ class DefaultRouterFacade(
         val content = inferenceResponse.text
 
         // Emit ROUTER turn event
-        emitChatTurn(sessionId, content, ChatTurnRole.ROUTER)
+        emitChatTurn(
+            sessionId, content, ChatTurnRole.ROUTER,
+            latencyMs = inferenceResponse.latencyMs,
+            tokensUsed = inferenceResponse.tokensUsed,
+        )
 
         if (mode == ChatMode.STEERING) {
             val validationError = validateSteering?.invoke(content)
@@ -160,7 +165,13 @@ class DefaultRouterFacade(
         return RouterResponse(content = content, steeringEmitted = (mode == ChatMode.STEERING))
     }
 
-    private suspend fun emitChatTurn(sessionId: SessionId, content: String, role: ChatTurnRole): String {
+    private suspend fun emitChatTurn(
+        sessionId: SessionId,
+        content: String,
+        role: ChatTurnRole,
+        latencyMs: Long? = null,
+        tokensUsed: TokenUsage? = null,
+    ): String {
         val turnId = UUID.randomUUID().toString()
         val now = Clock.System.now()
         eventStore.append(
@@ -179,6 +190,8 @@ class DefaultRouterFacade(
                     role = role,
                     content = content,
                     timestampMs = now.toEpochMilliseconds(),
+                    latencyMs = latencyMs,
+                    tokensUsed = tokensUsed,
                 ),
             ),
         )