fix: #38 EVENT_BUFFER 增加 MAX_BUFFER_SIZE=1000 上限

push_reading_event 时超出上限自动丢弃最旧事件，防止未导出事件堆积导致 OOM。

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

crates/zx_document_core/src/events.rs

@@ -4,12 +4,19 @@ use serde::{Deserialize, Serialize};
 
 use crate::progress::ReadingPosition;
 
+/// Maximum number of events before oldest are dropped to prevent unbounded memory growth.
+const MAX_BUFFER_SIZE: usize = 1000;
+
 // Global event buffer, protected by a Mutex for thread safety.
 static EVENT_BUFFER: Mutex<Vec<ReadingEvent>> = Mutex::new(Vec::new());
 
 /// Push a reading event into the global buffer.
+/// If the buffer exceeds MAX_BUFFER_SIZE, the oldest event is dropped.
 pub fn push_reading_event(event: ReadingEvent) {
     if let Ok(mut buf) = EVENT_BUFFER.lock() {
+        if buf.len() >= MAX_BUFFER_SIZE {
+            buf.remove(0);
+        }
         buf.push(event);
     }
 }
@@ -45,7 +52,7 @@ fn now_ms() -> i64 {
         .unwrap_or(0)
 }
 
-#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize, uniffi::Enum)]
 #[serde(tag = "type")]
 pub enum ReadingEvent {
     MaterialOpened {