fix(gost): restore single-node bypass and preserve FailFilter backoff

Address reviewer feedback from PR #14 fix:

1. Single-node case: Bypass selector/FailFilter to ensure availability.
   This matches upstream go-gost/x behavior - single nodes should always
   be attempted regardless of recent failures.

2. Multi-node case: Preserve FailFilter's backoff contract. When all nodes
   are marked as failed, return nil to signal 'no healthy nodes' rather
   than falling back to a known-bad node. This prevents hammering unhealthy
   nodes and respects the failTimeout window.

The handler's retry loop with ExcludeNodes context handles the multi-node
failover properly - this change ensures hop.Select() provides correct
information about node health status.

Fixes intermittent forwarding failures introduced by #14.

go-gost/x/hop/hop.go

@@ -201,11 +201,26 @@ func (p *chainHop) Select(ctx context.Context, opts ...hop.SelectOption) *chain.
 		return nodes[0]
 	}
 
-	// Always go through selector for proper FailFilter evaluation,
-	// even when there's only one node. This ensures failed nodes
-	// can be filtered out properly.
+	// For single-node case: bypass selector/FailFilter to ensure availability.
+	// The marker system still works for metrics, but we don't block requests
+	// based on recent failures - the connection will be attempted regardless.
+	// This matches upstream go-gost/x behavior.
+	if len(nodes) == 1 {
+		return nodes[0]
+	}
+
+	// Multi-node case: use selector with FailFilter for proper failover.
+	// FailFilter will exclude recently-failed nodes, allowing traffic to
+	// be routed to healthy alternatives.
 	if s := p.options.selector; s != nil {
-		return s.Select(ctx, nodes...)
+		if node := s.Select(ctx, nodes...); node != nil {
+			return node
+		}
+		// All nodes filtered out by FailFilter - all are marked as failed.
+		// Return nil to signal "no healthy nodes available" to the caller.
+		// The handler's retry loop will handle this appropriately.
+		log.Debugf("all %d nodes filtered out by FailFilter, no healthy nodes available", len(nodes))
+		return nil
 	}
 
 	// Fallback: return first node if no selector configured

go-gost/x/selector/filter.go

@@ -24,11 +24,12 @@ func FailFilter[T any](maxFails int, timeout time.Duration) selector.Filter[T] {
 }
 
 // Filter filters dead objects.
-// Note: We intentionally do NOT skip filtering when len(vs) <= 1.
-// This ensures that even a single dead node gets filtered out,
-// allowing the caller to know that no healthy nodes are available
-// and potentially trigger failover behavior.
+// For single-node case, skip filtering to ensure availability (matches upstream).
+// For multi-node case, filter out failed nodes to enable failover.
 func (f *failFilter[T]) Filter(ctx context.Context, vs ...T) []T {
+	if len(vs) <= 1 {
+		return vs
+	}
 	var l []T
 	for _, v := range vs {
 		maxFails := f.maxFails