fix(terminals): stop PTY output duplication on re-attach

Each open/reattach spawned a pump thread feeding pty_bridge[session]. The
broadcast hub added a subscriber per attach without dropping the previous one,
and the stale pump thread kept delivering (its send_output still succeeded
against the new channel) — so every byte was delivered twice (more after
further re-attaches). Accumulated on each tab/layout/agent switch; window
resize made it glaring.

- Broadcast::subscribe() is now single-consumer: a new subscription supersedes
  the previous one, ending the old pump thread's stream.
- PtyBridge gains a per-session generation; register() returns it and
  unregister_if(session, gen) only removes when still current, so a dying
  superseded thread can't tear down the channel that replaced it.
- All three attach sites (open/reattach/agent launch) use unregister_if.

Tests: 6 added (generation guard + single-consumer broadcast). Workspace green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/app-tauri/src/pty.rs

@@ -34,7 +34,11 @@ pub type PtyChunk = Vec<u8>;
 /// Thread-safe; cloned `Arc<PtyBridge>` is held in [`crate::state::AppState`].
 #[derive(Default)]
 pub struct PtyBridge {
-    channels: Mutex<HashMap<SessionId, Channel<PtyChunk>>>,
+    /// Per session: a monotonically-increasing **generation** plus the current
+    /// output channel. The generation distinguishes successive (re-)attaches so a
+    /// superseded pump thread can't tear down the channel of the attach that
+    /// replaced it (see [`PtyBridge::unregister_if`]).
+    channels: Mutex<HashMap<SessionId, (u64, Channel<PtyChunk>)>>,
 }
 
 impl PtyBridge {
@@ -46,21 +50,39 @@ impl PtyBridge {
         }
     }
 
-    /// Registers the output channel for a session (called when a terminal is
-    /// opened, from a `#[tauri::command]` that receives the `Channel` argument).
-    pub fn register(&self, session: SessionId, channel: Channel<PtyChunk>) {
+    /// Registers (or replaces) the output channel for a session and returns the
+    /// **generation** of this registration. Each call for a session bumps the
+    /// generation, so the caller's pump thread can later tear down *only its own*
+    /// registration via [`unregister_if`](Self::unregister_if).
+    pub fn register(&self, session: SessionId, channel: Channel<PtyChunk>) -> u64 {
         if let Ok(mut map) = self.channels.lock() {
-            map.insert(session, channel);
+            let gen = map.get(&session).map_or(0, |(g, _)| g.wrapping_add(1));
+            map.insert(session, (gen, channel));
+            gen
+        } else {
+            0
         }
     }
 
-    /// Removes a session's channel (terminal closed).
+    /// Removes a session's channel unconditionally (terminal explicitly closed).
     pub fn unregister(&self, session: &SessionId) {
         if let Ok(mut map) = self.channels.lock() {
             map.remove(session);
         }
     }
 
+    /// Removes a session's channel **only if** `gen` is still the current
+    /// generation. A pump thread calls this when its output stream ends: if the
+    /// session has since been re-attached (newer generation), this is a no-op, so
+    /// the dying thread never unregisters the live channel that superseded it.
+    pub fn unregister_if(&self, session: &SessionId, gen: u64) {
+        if let Ok(mut map) = self.channels.lock() {
+            if matches!(map.get(session), Some((g, _)) if *g == gen) {
+                map.remove(session);
+            }
+        }
+    }
+
     /// Forwards a chunk of output bytes to a session's channel.
     ///
     /// Returns `true` if the chunk was delivered, `false` if no channel is
@@ -71,7 +93,7 @@ impl PtyBridge {
             return false;
         };
         match map.get(session) {
-            Some(channel) => channel.send(chunk).is_ok(),
+            Some((_, channel)) => channel.send(chunk).is_ok(),
             None => false,
         }
     }