test(tui-03): add gap detection test for SnapshotPhaseReducer

apps/tui/src/test/kotlin/com/correx/apps/tui/reducer/SnapshotPhaseReducerTest.kt

@@ -33,6 +33,60 @@ class SnapshotPhaseReducerTest {
         assertTrue(state.sessions.sessions.isEmpty())
     }
 
+    @Test
+    fun `live event with sessionSequence at or below cursor is dropped`() {
+        val state = TuiState(snapshotPhase = false, cursors = mapOf("s1" to 5L))
+        val action = Action.ServerEventReceived(
+            ServerMessage.SessionStarted(
+                sessionId = SessionId("s1"),
+                workflowId = "wf",
+                sequence = 5L,
+                sessionSequence = 5L,
+            )
+        )
+
+        val result = SnapshotPhaseReducer.process(state, action)
+
+        assertTrue(result.actionsToDispatch.isEmpty())
+        assertEquals(5L, result.state.cursors["s1"])
+    }
+
+    @Test
+    fun `live event with sessionSequence one above cursor is applied and cursor advances`() {
+        val state = TuiState(snapshotPhase = false, cursors = mapOf("s1" to 5L))
+        val action = Action.ServerEventReceived(
+            ServerMessage.SessionStarted(
+                sessionId = SessionId("s1"),
+                workflowId = "wf",
+                sequence = 6L,
+                sessionSequence = 6L,
+            )
+        )
+
+        val result = SnapshotPhaseReducer.process(state, action)
+
+        assertEquals(listOf(action), result.actionsToDispatch)
+        assertEquals(6L, result.state.cursors["s1"])
+    }
+
+    @Test
+    fun `gap event is applied and cursor advances to incoming sequence`() {
+        val state = TuiState(snapshotPhase = false, cursors = mapOf("s1" to 5L))
+        val action = Action.ServerEventReceived(
+            ServerMessage.SessionStarted(
+                sessionId = SessionId("s1"),
+                workflowId = "wf",
+                sequence = 8L,
+                sessionSequence = 8L,
+            )
+        )
+
+        val result = SnapshotPhaseReducer.process(state, action)
+
+        assertEquals(listOf(action), result.actionsToDispatch)
+        assertEquals(8L, result.state.cursors["s1"])
+    }
+
     @Test
     fun `during snapshotPhase SessionSnapshot installs cursor and is forwarded to sub-reducers`() {
         val state = TuiState(snapshotPhase = true)