feat: FFI DTO V2 completion - add EventBufferStateV2 (DOC-FULL-007)

- 新增 EventBufferStateV2 结构体 (total/pending/exported/failed)
- 新增 get_event_buffer_state_v2() 方法
- FFI 导出 EventBufferStateV2 类型 + 方法
- FFI V2 exports 审计: 6 DTO + 9 方法全部到位
- V1 保留 deprecated，V2 接口显式带 V2 后缀

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

crates/zx_document_core/src/events_v2.rs

@@ -211,6 +211,28 @@ pub fn buffer_size_v2() -> usize {
     buffer().lock().map(|b| b.len()).unwrap_or(0)
 }
 
+/// Buffer state snapshot for diagnostic / debug use.
+#[derive(Debug, Clone, uniffi::Record)]
+pub struct EventBufferStateV2 {
+    pub total: u32,
+    pub pending: u32,
+    pub exported: u32,
+    pub failed: u32,
+}
+
+pub fn get_event_buffer_state_v2() -> EventBufferStateV2 {
+    match buffer().lock() {
+        Ok(buf) => {
+            let total = buf.len() as u32;
+            let pending = buf.iter().filter(|b| b.state == BufferedEventState::Pending).count() as u32;
+            let exported = buf.iter().filter(|b| b.state == BufferedEventState::Exported).count() as u32;
+            let failed = buf.iter().filter(|b| b.state == BufferedEventState::Failed).count() as u32;
+            EventBufferStateV2 { total, pending, exported, failed }
+        }
+        Err(_) => EventBufferStateV2 { total: 0, pending: 0, exported: 0, failed: 0 },
+    }
+}
+
 pub fn clear_all_events_v2() {
     if let Ok(mut buf) = buffer().lock() {
         buf.clear();
@@ -608,6 +630,33 @@ mod tests {
         teardown_session(&sid);
     }
 
+    #[test]
+    fn test_get_event_buffer_state() {
+        let _guard = lock_test();
+        clear_all_events_v2();
+        let sid = setup_session("mat_state");
+
+        let s0 = get_event_buffer_state_v2();
+        assert_eq!(s0.total, 0);
+
+        push_material_opened_v2(&sid, "mat_state", 1000).unwrap();
+        let s1 = get_event_buffer_state_v2();
+        assert_eq!(s1.total, 1);
+        assert_eq!(s1.pending, 1);
+
+        let exported = export_pending_events_v2(100, 2000);
+        assert!(!exported.is_empty());
+        let s2 = get_event_buffer_state_v2();
+        assert_eq!(s2.exported, 1);
+        assert_eq!(s2.pending, 0);
+
+        ack_events_v2(&[exported[0].event_id.clone()]);
+        let s3 = get_event_buffer_state_v2();
+        assert_eq!(s3.total, 0);
+
+        teardown_session(&sid);
+    }
+
     #[test]
     fn test_delta_semantics_per_event_type() {
         let _guard = lock_test();

crates/zx_document_ffi/src/lib.rs

@@ -14,7 +14,7 @@ pub use zx_document_core::events::ReadingEvent;
 #[allow(deprecated)]
 pub use zx_document_core::reading_material::ReadingMaterialRef;
 pub use zx_document_core::reading_material::ReadingMaterialRefV2;
-pub use zx_document_core::events_v2::{ReadingEventV2, ReadingEventTypeV2};
+pub use zx_document_core::events_v2::{ReadingEventV2, ReadingEventTypeV2, EventBufferStateV2};
 pub use zx_document_core::epub::{EpubMetadata, EpubChapter};
 pub use zx_document_core::office::{OfficePreviewConfig, OfficePreviewStrategy};
 pub use zx_document_core::pdf::{PdfMetadata, PdfPageText};
@@ -114,6 +114,11 @@ fn reload_stale_events_v2() -> u32 {
     zx_document_core::events_v2::reload_stale_events_v2()
 }
 
+#[uniffi::export]
+fn get_event_buffer_state_v2() -> EventBufferStateV2 {
+    zx_document_core::events_v2::get_event_buffer_state_v2()
+}
+
 #[uniffi::export]
 fn ack_events_v2(event_ids: Vec<String>) -> u32 {
     zx_document_core::events_v2::ack_events_v2(&event_ids)