feat(events): add RouterNarrationEvent with polymorphic registration and round-trip test

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

@@ -50,3 +50,16 @@ data class ContextTruncatedEvent(
     val truncatedLayers: List<String>,
     val timestampMs: Long,
 ) : EventPayload
+
+@Serializable
+@SerialName("RouterNarration")
+data class RouterNarrationEvent(
+    val sessionId: SessionId,
+    val narrationId: String,
+    val trigger: String,
+    val stageId: String? = null,
+    val content: String,
+    val latencyMs: Long? = null,
+    val tokensUsed: TokenUsage? = null,
+    val timestampMs: Long,
+) : EventPayload

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

@@ -9,6 +9,7 @@ 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.RouterNarrationEvent
 import com.correx.core.events.events.SessionWorkspaceBoundEvent
 import com.correx.core.events.events.ContextTruncatedEvent
 import com.correx.core.events.events.EventPayload
@@ -80,6 +81,7 @@ val eventModule = SerializersModule {
         subclass(RiskAssessedEvent::class)
         subclass(ChatSessionStartedEvent::class)
         subclass(ChatTurnEvent::class)
+        subclass(RouterNarrationEvent::class)
         subclass(SessionWorkspaceBoundEvent::class)
         subclass(L3MemoryRetrievedEvent::class)
         subclass(ContextTruncatedEvent::class)

core/events/src/test/kotlin/com/correx/core/events/serialization/RouterNarrationEventSerializationTest.kt

@@ -0,0 +1,45 @@
+package com.correx.core.events.serialization
+
+import com.correx.core.events.events.EventPayload
+import com.correx.core.events.events.RouterNarrationEvent
+import com.correx.core.events.types.SessionId
+import com.correx.core.inference.TokenUsage
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class RouterNarrationEventSerializationTest {
+
+    private val sample = RouterNarrationEvent(
+        sessionId = SessionId("sess-1"),
+        narrationId = "narr-42",
+        trigger = "stage_start",
+        stageId = "stage-1",
+        content = "Router chose the default path.",
+        latencyMs = 123L,
+        tokensUsed = TokenUsage(promptTokens = 10, completionTokens = 5),
+        timestampMs = 1_700_000_000_000L,
+    )
+
+    @Test
+    fun `round-trips as polymorphic EventPayload`() {
+        val encoded = eventJson.encodeToString(EventPayload.serializer(), sample)
+        assertTrue(encoded.contains("\"type\":\"RouterNarration\""), "SerialName must be present: $encoded")
+        val decoded = eventJson.decodeFromString(EventPayload.serializer(), encoded)
+        assertEquals(sample, decoded)
+    }
+
+    @Test
+    fun `round-trips with optional fields absent`() {
+        val minimal = RouterNarrationEvent(
+            sessionId = SessionId("sess-2"),
+            narrationId = "narr-0",
+            trigger = "manual",
+            content = "Minimal narration.",
+            timestampMs = 1_000L,
+        )
+        val encoded = eventJson.encodeToString(EventPayload.serializer(), minimal)
+        val decoded = eventJson.decodeFromString(EventPayload.serializer(), encoded)
+        assertEquals(minimal, decoded)
+    }
+}