style: apply cargo fmt across the crate (A20)

The repo never enforced rustfmt, so formatting had drifted broadly. This is a
single mechanical `cargo fmt` pass over the whole crate (no behavioral change;
lib suite green, 493 passed). Going forward fmt should be enforced (planned CI
fmt --check step). Part of the 0.6.1 hygiene pass.

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

tests/file_transfer_loopback.rs

@@ -21,7 +21,7 @@ use iroh::endpoint::presets;
 use iroh::protocol::Router;
 use iroh::{Endpoint, RelayMode};
 
-use peerspeak::files::{ChatAttachment, AttachmentKind};
+use peerspeak::files::{AttachmentKind, ChatAttachment};
 use peerspeak::network::NetworkTransport;
 use peerspeak::network::iroh_impl::{FileRouter, IrohTransport};
 use peerspeak::protocol::FILES_ALPN;
@@ -52,7 +52,12 @@ async fn spawn_node() -> Node {
         .accept(FILES_ALPN, file_router)
         .spawn();
 
-    Node { endpoint, transport, _router: router, lookup }
+    Node {
+        endpoint,
+        transport,
+        _router: router,
+        lookup,
+    }
 }
 
 /// A pseudo-random-ish multi-megabyte payload spanning many QUIC packets, so a
@@ -86,7 +91,9 @@ async fn loopback_attachment_round_trips_intact() {
 
     let blob = big_blob();
     let id = [42u8; 32];
-    server.transport.serve_attachment(id, Arc::new(blob.clone()));
+    server
+        .transport
+        .serve_attachment(id, Arc::new(blob.clone()));
 
     let att = ChatAttachment {
         name: "exterior-landscape.jpg".to_string(),