refactor(peer): extract peer startup task spawning

The peer runtime used to spawn each long-running service inline inside
run_peer. That made the startup path harder to scan because service names,
clone setup, and task error handling were interleaved with the command loop.

Move the task wrappers into a startup module and leave run_peer as the
lifecycle overview: create shared context, start services, handle commands,
then send shutdown goodbyes. The spawned services and their error handling are
unchanged; only the ownership plumbing moved into named helpers.

Test Plan:
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo +nightly fmt

Refs: none

crates/lanspread-peer/src/services.rs

@@ -1,7 +1,8 @@
 //! Long-running peer services.
 //!
 //! Each submodule owns one runtime concern. The public surface intentionally
-//! stays small because `lib.rs` only needs to start these four background tasks.
+//! stays small because peer startup only needs to start these four background
+//! tasks.
 
 mod advertise;
 mod discovery;