fix: complete all P2 audit findings — dead code, NPE guard, hygiene

P2-1: Guard SessionId NPE with safe-call in SessionsReducer
P2-2: Remove 16 dead event classes + reducer branches; replace with live orchestration events in tests
P2-3: Standardize ChatInput divergences — guard CancelSession, single-arg ProtocolError
P2-4: Remove dead TuiToolRecord.diff and ToolDisplayStatus.REQUESTED
P2-5: Remove dead streamLive from SessionEventBridge
P2-6: Extract SessionsReducerContext data class for 6+ param method
P2-7: Replace StageId("none") sentinel with TypeId.NONE
P2-9: Add TypeId.random() factory on all type-alias IDs
P2-10: Add KDoc on schemaVersion documenting reserved-for-migration
P2-8: Verified RouterReducer string-template already correct (TypeId value class toString)

testing/transitions/src/test/kotlin/TransitionEventSerializationTest.kt

@@ -1,7 +1,6 @@
 import com.correx.core.events.events.EventPayload
 import com.correx.core.events.events.StageCompletedEvent
 import com.correx.core.events.events.StageFailedEvent
-import com.correx.core.events.events.StageStartedEvent
 import com.correx.core.events.events.TransitionExecutedEvent
 import com.correx.core.events.serialization.JsonEventSerializer
 import com.correx.core.events.types.SessionId
@@ -13,10 +12,11 @@ import org.junit.jupiter.api.Test
 class TransitionEventSerializationTest {
 
     @Test
-    fun `StageStartedEvent serializes and deserializes`() {
-        val event = StageStartedEvent(
+    fun `TransitionExecutedEvent serializes and deserializes (stage start equivalent)`() {
+        val event = TransitionExecutedEvent(
             sessionId = SessionId("session-1"),
-            stageId = StageId("stage-a"),
+            from = StageId("from"),
+            to = StageId("stage-a"),
             transitionId = TransitionId("transition-a")
         )