fix(kcp): prevent zero-value override of window/buffer params causing 10x perf drop

The metadata parser unconditionally assigned GetInt results (which
return 0 for missing keys) to all KCP config fields, overwriting
defaults like SndWnd=1024, RcvWnd=1024, StreamBuf=2097152 with 0.

KCP's WndSize only applies positive values, so snd_wnd=0 caused
the write path condition waitsnd < s.kcp.snd_wnd to never succeed,
effectively stalling throughput at ~10Mbps instead of 400Mbps+.

- Only apply metadata values when the key actually exists (IsExists check)
- Use Clone() instead of sharing the global DefaultConfig pointer
- Add Config.Clone() deep-copy method

go-gost/x/internal/util/kcp/config.go

@@ -89,6 +89,15 @@ func ParseFromFile(filename string) (*Config, error) {
 	return config, nil
 }
 
+// Clone returns a deep copy of the Config.
+func (c *Config) Clone() *Config {
+	if c == nil {
+		return nil
+	}
+	clone := *c
+	return &clone
+}
+
 // Init initializes the KCP config.
 func (c *Config) Init() {
 	switch c.Mode {