fix(direct): guard nil dialer and nil logger in Connect, add doc comments

Add nil checks for connect-options dialer and logger to prevent
nil-pointer panics when Connect is called without a DialerConnectOption
or LoggerOption. Add package and NewConnector godoc comments.

Add 10 unit tests covering construction, Init metadata parsing,
Connect success/reject/dial-error paths, nil dialer, nil logger,
and the reject conn stub.

connector/direct/connector.go

@@ -1,7 +1,12 @@
+// Package direct implements a direct (transparent) connector that establishes
+// connections to destination addresses using the dialer provided via connect
+// options. It also supports a "reject" action that returns a dead connection
+// (reads return io.EOF, writes return io.ErrClosedPipe) without dialing.
 package direct
 
 import (
 	"context"
+	"errors"
 	"net"
 
 	"github.com/go-gost/core/connector"
@@ -20,6 +25,7 @@ type directConnector struct {
 	options connector.Options
 }
 
+// NewConnector creates a direct connector with the given options.
 func NewConnector(opts ...connector.Option) connector.Connector {
 	options := connector.Options{}
 	for _, opt := range opts {
@@ -45,27 +51,33 @@ func (c *directConnector) Connect(ctx context.Context, _ net.Conn, network, addr
 		return &conn{}, nil
 	}
 
+	if cOpts.Dialer == nil {
+		return nil, errors.New("direct: missing dialer in connect options")
+	}
+
 	conn, err := cOpts.Dialer.Dial(ctx, network, address)
 	if err != nil {
 		return nil, err
 	}
 
-	var localAddr, remoteAddr string
-	if addr := conn.LocalAddr(); addr != nil {
-		localAddr = addr.String()
-	}
-	if addr := conn.RemoteAddr(); addr != nil {
-		remoteAddr = addr.String()
-	}
+	if c.options.Logger != nil {
+		var localAddr, remoteAddr string
+		if addr := conn.LocalAddr(); addr != nil {
+			localAddr = addr.String()
+		}
+		if addr := conn.RemoteAddr(); addr != nil {
+			remoteAddr = addr.String()
+		}
 
-	log := c.options.Logger.WithFields(map[string]any{
-		"remote":  remoteAddr,
-		"local":   localAddr,
-		"network": network,
-		"address": address,
-		"sid":     string(ctxvalue.SidFromContext(ctx)),
-	})
-	log.Debugf("connect %s/%s", address, network)
+		log := c.options.Logger.WithFields(map[string]any{
+			"remote":  remoteAddr,
+			"local":   localAddr,
+			"network": network,
+			"address": address,
+			"sid":     string(ctxvalue.SidFromContext(ctx)),
+		})
+		log.Debugf("connect %s/%s", address, network)
+	}
 
 	return conn, nil
 }