fix(gost): implement failover for multi-node forwarding rules (#12)

When a forwarding rule has multiple backend nodes configured, the first
node failure would cause the entire forward to fail instead of trying
the next available node.

Root causes fixed:
- FailFilter skipped filtering when only 1 node remained
- hop.Select() bypassed selector for single-node hops
- Handlers only attempted one node before giving up

Changes:
- selector/filter.go: Remove len<=1 early return, always filter failed nodes
- hop/hop.go: Remove single-node bypass, add ExcludeNodes context support
- ctx/value.go: Add ContextWithExcludeNodes/ExcludeNodesFromContext helpers
- handler/forward/local: Add maxRetries config, implement retry loop
- handler/forward/remote: Add maxRetries config, implement retry loop
- forwarder/sniffer.go: Add retry logic to dial() and dialTLS()

Closes #12

go-gost/x/selector/filter.go

@@ -5,8 +5,8 @@ import (
 	"time"
 
 	"github.com/go-gost/core/metadata"
-	mdutil "github.com/go-gost/x/metadata/util"
 	"github.com/go-gost/core/selector"
+	mdutil "github.com/go-gost/x/metadata/util"
 )
 
 type failFilter[T any] struct {
@@ -24,10 +24,11 @@ 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.
 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