fix(peer): preserve monitor backoff per root

Track the last attempted configured root separately from the last successful snapshot. A root switch clears backoff once; repeated budget failures on that same root now retain exponential delay.

Test Plan:
- just test
- just clippy
- repeated failing-root backoff regression
- git diff --check

crates/lanspread-peer/src/services/local_monitor.rs

@@ -132,9 +132,18 @@ impl PollBudget {
 struct PollBackoff {
     failures: u32,
     retry_at: Option<Instant>,
+    observed_root: Option<PathBuf>,
 }
 
 impl PollBackoff {
+    fn observe_root(&mut self, root: &Path) {
+        if self.observed_root.as_deref() == Some(root) {
+            return;
+        }
+        self.observed_root = Some(root.to_path_buf());
+        self.record_success();
+    }
+
     fn should_attempt(&self, now: Instant) -> bool {
         self.retry_at.is_none_or(|retry_at| now >= retry_at)
     }
@@ -264,14 +273,8 @@ async fn poll_local_game_changes(
     previous: &mut Option<PollSnapshot>,
     backoff: &mut PollBackoff,
 ) {
-    let configured_game_dir = ctx.game_dir.read().await;
-    if previous
-        .as_ref()
-        .is_some_and(|snapshot| snapshot.game_dir != *configured_game_dir)
-    {
-        backoff.record_success();
-    }
-    drop(configured_game_dir);
+    let configured_game_dir = ctx.game_dir.read().await.clone();
+    backoff.observe_root(&configured_game_dir);
 
     let now = Instant::now();
     if !backoff.should_attempt(now) {
@@ -733,6 +736,27 @@ mod tests {
         assert_eq!(backoff.failures, 0);
     }
 
+    #[test]
+    fn root_change_resets_backoff_only_once_until_a_snapshot_succeeds() {
+        let now = Instant::now();
+        let mut backoff = PollBackoff::default();
+        let first_root = Path::new("/first");
+        let failing_root = Path::new("/oversized");
+
+        backoff.observe_root(first_root);
+        backoff.record_failure(now);
+        backoff.observe_root(failing_root);
+        assert!(backoff.should_attempt(now));
+
+        let delay = backoff.record_failure(now);
+        backoff.observe_root(failing_root);
+        assert!(
+            !backoff.should_attempt(now),
+            "re-observing the same failing root must preserve its backoff"
+        );
+        assert!(backoff.should_attempt(now + delay));
+    }
+
     #[test]
     fn snapshot_diff_detects_game_change_and_disappearance() {
         let temp = TempDir::new("lanspread-local-monitor-change");