feat(events,server): ExecutionPlanRejectedEvent — freestyle compile failure visible in log

Both silent-failure branches in FreestyleDriver.lockAndRun now emit
ExecutionPlanRejectedEvent (source=compile or missing_content) before
returning, satisfying invariant #1. Event registered in eventModule for
correct polymorphic serialization.

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

@@ -20,3 +20,17 @@ data class ExecutionPlanLockedEvent(
     val stageIds: List<String>,
     val startStageId: String,
 ) : EventPayload
+
+/**
+ * The architect's execution_plan could not become the phase-2 graph: content missing,
+ * compile/validation failure, or operator rejection at the plan-review gate. Terminal for
+ * the freestyle run unless re-planned; recorded so replay explains why phase 2 never ran.
+ */
+@Serializable
+@SerialName("ExecutionPlanRejected")
+data class ExecutionPlanRejectedEvent(
+    val sessionId: SessionId,
+    val reason: String,
+    /** "compile" | "missing_content" | "operator" */
+    val source: String,
+) : EventPayload

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

@@ -16,6 +16,7 @@ import com.correx.core.events.events.SessionWorkspaceBoundEvent
 import com.correx.core.events.events.ContextTruncatedEvent
 import com.correx.core.events.events.EventPayload
 import com.correx.core.events.events.ExecutionPlanLockedEvent
+import com.correx.core.events.events.ExecutionPlanRejectedEvent
 import com.correx.core.events.events.JournalCompactedEvent
 import com.correx.core.events.events.FileWrittenEvent
 import com.correx.core.events.events.L3MemoryRetrievedEvent
@@ -102,6 +103,7 @@ val eventModule = SerializersModule {
         subclass(L3MemoryRetrievedEvent::class)
         subclass(ContextTruncatedEvent::class)
         subclass(ExecutionPlanLockedEvent::class)
+        subclass(ExecutionPlanRejectedEvent::class)
         subclass(JournalCompactedEvent::class)
     }
 }

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

@@ -0,0 +1,24 @@
+package com.correx.core.events.serialization
+
+import com.correx.core.events.events.EventPayload
+import com.correx.core.events.events.ExecutionPlanRejectedEvent
+import com.correx.core.events.types.SessionId
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class ExecutionPlanRejectedEventSerializationTest {
+
+    @Test
+    fun `ExecutionPlanRejectedEvent round-trips as polymorphic EventPayload`() {
+        val sample: EventPayload = ExecutionPlanRejectedEvent(
+            sessionId = SessionId("s"),
+            reason = "plan failed to compile: Unknown artifact kind",
+            source = "compile",
+        )
+        val encoded = eventJson.encodeToString(EventPayload.serializer(), sample)
+        assertTrue(encoded.contains("\"type\":\"ExecutionPlanRejected\""), "SerialName must be present: $encoded")
+        val decoded = eventJson.decodeFromString(EventPayload.serializer(), encoded)
+        assertEquals(sample, decoded)
+    }
+}