test(peer): consolidate temp dir helper

Move the repeated test TempDir implementations into a single peer
test_support module. The shared helper keeps the existing automatic cleanup
behavior and uses an atomic suffix plus timestamp so parallel tests do not
collide on the same path.

This is intentionally limited to test hygiene. It does not change the
availability model, split download.rs, or touch production scan/install
behavior beyond importing the shared helper from test modules.

Test Plan:
- git diff --check
- just fmt
- just clippy
- just test

Follow-up-Plan: FOLLOW_UP_2.md

crates/lanspread-peer/src/lib.rs

@@ -29,6 +29,8 @@ mod peer_db;
 mod remote_peer;
 mod services;
 mod startup;
+#[cfg(test)]
+mod test_support;
 
 // =============================================================================
 // Public re-exports