fix: premature socket teardown on CLOSE + duplicate CLOSE echo

Two bugs when upstream closes:

1. Data loss: gatuna received CLOSE from gatunad and immediately
   RST'd the socket to curl while the drain pump still had queued
   payloads in _deliverChannel. Fix: OnRemoteClose() completes the
   channel writer and cancels the read pump, but lets the drain pump
   finish naturally. Drain pump no longer takes _cts.Token — it stops
   on channel completion and closes the socket gracefully (FIN, OS
   flushes in background).

2. Duplicate CLOSE: gatuna echoed CLOSE back to gatunad (server
   already knows — it initiated). SendClose() now guarded by
   _closeSent flag. OnRemoteClose() never sends CLOSE at all.

Server-side: remove session from store before sending CLOSE so the
retransmit timer stops before CLOSE is queued. Only log CLOSE if
the session actually existed (dedup).

gatunad/src/main.rs

@@ -224,8 +224,9 @@ async fn handle_frame(
             }
         }
         Frame::Close { session_id, reason: _ } => {
-            info!("CLOSE session {session_id} from {src}");
-            store.lock().expect("store poisoned").remove(&session_id);
+            if store.lock().expect("store poisoned").remove(&session_id).is_some() {
+                info!("CLOSE session {session_id} from {src}");
+            }
         }
         Frame::Ping { nonce } => {
             let pong = Frame::Pong { nonce };

gatunad/src/session.rs

@@ -275,12 +275,12 @@ pub fn spawn_pump(
                 }
 
                 if should_close {
+                    store.lock().expect("store poisoned").remove(&session_id);
                     let close = Frame::Close {
                         session_id,
                         reason: Some(REASON_MAX_RETRIES),
                     };
                     let _ = tx.try_send((peer_mac, close.encode()));
-                    store.lock().expect("store poisoned").remove(&session_id);
                     break;
                 }
             }
@@ -330,8 +330,8 @@ pub fn spawn_pump(
                 break;
             }
         }
+        store.lock().expect("store poisoned").remove(&session_id);
         let close = Frame::Close { session_id, reason: None };
         let _ = tx.send((peer_mac, close.encode())).await;
-        store.lock().expect("store poisoned").remove(&session_id);
     });
 }