refactor(peer-cli): split download measurement event handlers

Extract the download-begin and chunk-finished measurement bookkeeping out of the
main peer-cli event reducer. This keeps the S37 throughput reporting behavior
unchanged while bringing the reducer back under the pedantic clippy line-count
threshold.

Test Plan:
- just fmt
- just clippy
- just test

Refs: S37 download throughput measurement harness.

crates/lanspread-peer-cli/src/main.rs

@@ -450,17 +450,7 @@ async fn update_state_from_event(shared: &SharedState, event: PeerEvent) -> (&'s
                 json!({"game_id": id, "file_descriptions": file_descriptions}),
             )
         }
-        PeerEvent::DownloadGameFilesBegin { id } => {
+        PeerEvent::DownloadGameFilesBegin { id } => download_begin_event(shared, id).await,
-            shared.state.write().await.downloads.insert(
-                id.clone(),
-                DownloadMeasurement {
-                    started_at: Instant::now(),
-                    bytes: 0,
-                    chunks: 0,
-                },
-            );
-            ("download-begin", json!({"game_id": id}))
-        }
         PeerEvent::DownloadGameFileChunkFinished {
             id,
             peer_addr,
@@ -468,20 +458,8 @@ async fn update_state_from_event(shared: &SharedState, event: PeerEvent) -> (&'s
             offset,
             length,
         } => {
-            if let Some(measurement) = shared.state.write().await.downloads.get_mut(&id) {
+            download_chunk_finished_event(shared, id, peer_addr, relative_path, offset, length)
-                measurement.bytes = measurement.bytes.saturating_add(length);
+                .await
-                measurement.chunks = measurement.chunks.saturating_add(1);
-            }
-            (
-                "download-chunk-finished",
-                json!({
-                    "game_id": id,
-                    "peer_addr": peer_addr.to_string(),
-                    "relative_path": relative_path,
-                    "offset": offset,
-                    "length": length,
-                }),
-            )
         }
         PeerEvent::DownloadGameFilesFinished { id } => {
             download_terminal_event(shared, "download-finished", id).await
@@ -521,6 +499,43 @@ fn game_id_event(kind: &'static str, id: String) -> (&'static str, Value) {
     (kind, json!({"game_id": id}))
 }
 
+async fn download_begin_event(shared: &SharedState, id: String) -> (&'static str, Value) {
+    shared.state.write().await.downloads.insert(
+        id.clone(),
+        DownloadMeasurement {
+            started_at: Instant::now(),
+            bytes: 0,
+            chunks: 0,
+        },
+    );
+    game_id_event("download-begin", id)
+}
+
+async fn download_chunk_finished_event(
+    shared: &SharedState,
+    id: String,
+    peer_addr: SocketAddr,
+    relative_path: String,
+    offset: u64,
+    length: u64,
+) -> (&'static str, Value) {
+    if let Some(measurement) = shared.state.write().await.downloads.get_mut(&id) {
+        measurement.bytes = measurement.bytes.saturating_add(length);
+        measurement.chunks = measurement.chunks.saturating_add(1);
+    }
+
+    (
+        "download-chunk-finished",
+        json!({
+            "game_id": id,
+            "peer_addr": peer_addr.to_string(),
+            "relative_path": relative_path,
+            "offset": offset,
+            "length": length,
+        }),
+    )
+}
+
 async fn download_terminal_event(
     shared: &SharedState,
     kind: &'static str,