perf: 消息内存按字节预算，杜绝大报文流撑爆内存

- 新增 MessageRing：条数上限 + 每连接 32MB 字节预算双重驱逐
  （从最旧丢弃，均摊 O(1)）；预分配容量按设置钳制在 512
- 单条 payload 保留上限 256KB → 128KB；MqttRecord::retained_bytes
  计入文本与原始字节，供预算驱逐与资源监控展示
- 清空消息统一走 clear_messages；设置页标注字节预算兜底说明
- 新增单元测试：条数驱逐、字节预算、超预算单条保留、驱逐顺序

gpui-app/src/ui/app.rs

@@ -24,8 +24,8 @@ use gpui_kit::{
 use crate::aliyun::AliyunPreset;
 use crate::model::{
     AppSettings, ConnectionConfig, ConnectionStatus, Direction, GlobalVariable, LogEntry,
-    LogLevel, MqttRecord, PublishParams, PublishPreset, SubscribeOptions, Subscription,
-    ThemeModePref, retained_payload,
+    LogLevel, MqttRecord, MessageRing, PublishParams, PublishPreset, SubscribeOptions,
+    Subscription, ThemeModePref, retained_payload,
 };
 use crate::mqtt::{EngineEvent, MqttEngine};
 use crate::store::Storage;
@@ -84,8 +84,8 @@ pub struct MqttXApp {
     // ── 运行时状态 ──
     pub statuses: HashMap<String, ConnectionStatus>,
     pub errors: HashMap<String, String>,
-    /// 每连接消息环形缓冲：键为驻留后的连接 ID，容量受 max_messages 约束
-    pub messages: HashMap<Arc<str>, VecDeque<MqttRecord>>,
+    /// 每连接消息环形缓冲：键为驻留后的连接 ID，条数 + 字节预算双重约束
+    pub messages: HashMap<Arc<str>, MessageRing>,
     pub logs: VecDeque<LogEntry>,
     pub seq: u64,
 
@@ -473,16 +473,16 @@ impl MqttXApp {
     }
 
     fn push_message(&mut self, connection_id: Arc<str>, record: MqttRecord) {
-        let cap = self.settings.max_messages.max(100);
-        let list = self
-            .messages
+        let cap = self.settings.max_messages;
+        self.messages
             .entry(connection_id)
-            .or_insert_with(|| VecDeque::with_capacity(cap));
-        // 环形缓冲：满则丢最旧，均摊 O(1)（原来的 Vec::drain 是 O(n) 搬移）
-        while list.len() >= cap {
-            list.pop_front();
-        }
-        list.push_back(record);
+            .or_insert_with(|| MessageRing::new(cap))
+            .push(record, cap);
+    }
+
+    /// 清空指定连接的消息（内存缓冲随 drop 释放）。
+    pub fn clear_messages(&mut self, id: &str) {
+        self.messages.remove(id);
     }
 
     fn push_log(&mut self, entry: LogEntry) {
@@ -603,7 +603,7 @@ impl MqttXApp {
         self.engine.close(id, false);
         self.connections.retain(|c| c.id != id);
         self.subscriptions.retain(|s| s.connection_id != id);
-        self.messages.remove(id);
+        self.clear_messages(id);
         self.statuses.remove(id);
         self.errors.remove(id);
         self.views.remove(id);

gpui-app/src/ui/connection_view.rs

@@ -1765,7 +1765,7 @@ impl ConnectionView {
                                                 view.detail_format = DetailFormat::Auto;
                                                 if let Some(app) = view.app.upgrade() {
                                                     app.update(cx, |app, cx| {
-                                                        app.messages.remove(clear_id.as_str());
+                                                        app.clear_messages(&clear_id);
                                                         cx.notify();
                                                     });
                                                 }

gpui-app/src/ui/settings_dialog.rs

@@ -17,7 +17,7 @@ use gpui_kit::{
     Styled as _, Window,
 };
 
-use crate::model::{AppSettings, ThemeModePref};
+use crate::model::{AppSettings, ThemeModePref, MAX_CONNECTION_MESSAGE_BYTES};
 use crate::mqtt::MqttEngine;
 use crate::ui::app::MqttXApp;
 use crate::ui::widgets::{field, make_select, OptionDelegate};
@@ -388,6 +388,15 @@ impl Render for SettingsDialog {
                 "每条连接内存中保留的消息条数（100~100000）",
                 Input::new(&self.max_messages),
             ))
+            .child(
+                div()
+                    .text_xs()
+                    .text_color(cx.theme().muted_foreground)
+                    .child(format!(
+                        "另有每连接 {}MB 的字节预算兜底：大报文流会优先按字节从最旧开始驱逐",
+                        MAX_CONNECTION_MESSAGE_BYTES / (1024 * 1024)
+                    )),
+            )
             .child(
                 h_flex()
                     .justify_between()