feat: ReadingEvent V2 delta semantics verification (DOC-FULL-003)

- 新增 test_delta_semantics_per_event_type: 验证所有 5 种事件类型的
  activeSecondsDelta 均为增量语义（非累计值）
- ReadingEventV2 + ReadingEventTypeV2 已有完整实现（610行/22测试）
- No readingTargetType/userId/knowledgeBaseId（编译时保证）

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/zx_document_core/src/events_v2.rs

@@ -607,4 +607,35 @@ mod tests {
         }
         teardown_session(&sid);
     }
+
+    #[test]
+    fn test_delta_semantics_per_event_type() {
+        let _guard = lock_test();
+        let sid = setup_session("mat_delta");
+
+        // material_opened: delta must be 0
+        let e1 = push_material_opened_v2(&sid, "mat_delta", 1000).unwrap();
+        assert_eq!(e1.active_seconds_delta, 0);
+
+        // position_changed: delta must be 0
+        let e2 = push_position_changed_v2(&sid, "mat_delta", ReadingPosition::Unknown, 2000).unwrap();
+        assert_eq!(e2.active_seconds_delta, 0);
+
+        // marked_as_read: delta must be 0
+        let e3 = push_marked_as_read_v2(&sid, "mat_delta", 3000).unwrap();
+        assert_eq!(e3.active_seconds_delta, 0);
+
+        // heartbeat: delta is the caller-provided active_seconds_delta (non-zero)
+        let e4 = push_heartbeat_v2(&sid, "mat_delta", 15, None, 4000).unwrap();
+        assert_eq!(e4.active_seconds_delta, 15);
+
+        // material_closed: delta is the residual active time at close
+        let e5 = push_material_closed_v2(&sid, "mat_delta", 5, 5000).unwrap();
+        assert_eq!(e5.active_seconds_delta, 5);
+
+        // Verify all deltas are NOT accumulated — they are per-event increments
+        assert_eq!(e1.active_seconds_delta + e2.active_seconds_delta + e3.active_seconds_delta + e4.active_seconds_delta + e5.active_seconds_delta, 20);
+
+        teardown_session(&sid);
+    }
 }