fix(peer): coalesce full UI view publications

Keep one queued and one replaceable pending snapshot for remote-library and Call-to-Play views while preserving lifecycle-event FIFO delivery. Generation barriers and fenced drains prevent stale nonempty views from crossing disable or acknowledgement boundaries.

Test Plan:
- just test
- just clippy
- focused burst, lifecycle ordering, fence, repeated-barrier, and stale-view tests
- independent ordering review
- git diff --check

crates/lanspread-peer/src/download/orchestrator.rs

@@ -9,7 +9,7 @@ use std::{
 use futures::stream::FuturesUnordered;
 use lanspread_db::content_manifest::ContentId;
 use lanspread_proto::PeerEndpoint;
-use tokio::sync::mpsc::UnboundedSender;
+use crate::PeerEventSender;
 use tokio_util::sync::CancellationToken;
 
 use super::{
@@ -153,7 +153,7 @@ pub(crate) struct DownloadGameRequest<'a> {
     pub(crate) sources: &'a [PeerEndpoint],
     pub(crate) content_id: ContentId,
     pub(crate) quarantine: &'a ContentQuarantine,
-    pub(crate) tx_notify_ui: UnboundedSender<PeerEvent>,
+    pub(crate) tx_notify_ui: PeerEventSender,
     pub(crate) cancel_token: CancellationToken,
     pub(crate) quic: QuicConnector,
 }
@@ -372,7 +372,7 @@ struct TransferContext<'a> {
     sources: &'a [PeerEndpoint],
     content_id: ContentId,
     quarantine: &'a ContentQuarantine,
-    tx_notify_ui: &'a UnboundedSender<PeerEvent>,
+    tx_notify_ui: &'a PeerEventSender,
     cancel_token: &'a CancellationToken,
     quic: &'a QuicConnector,
     version_buffer: Arc<VersionIniBuffer>,