refactor: remove link layer

internal/link and internal/link/direct were a single-implementation
abstraction layer where directLink mechanically proxied every method to
transport.Transport — only Features() lived above transport.Transport,
and even that was a Features() alias. Six layers of plumbing for zero
behavioural value.

Drop the layer entirely:
- muxconn.Conn now takes a transport.Transport directly.
- server.Server and client.Client store transport.Transport, call
  transport.New, and expose Features() through transport.Transport's
  built-in method.
- server.Config and client.Config lose their Link string field.
- session.Config loses Link + validateLink + ErrLinkRequired/ErrUnsupportedLink.
- config.File and config.Profile lose the link YAML key.
- pkg/olcrtc/tunnel.Config loses Link.
- mobile drops defaultLink, SetLink, and mobileConfig.link.

Two e2e tests that exercised link.New directly are renamed to call
transport.New (TestTransportCreatesAllProviderTransportCombinations and
TestTransportConnectsFastProviderTransportMatrix); behaviour is unchanged.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/server/server.go

@@ -17,7 +17,6 @@ import (
 	"github.com/openlibrecommunity/olcrtc/internal/control"
 	"github.com/openlibrecommunity/olcrtc/internal/crypto"
 	"github.com/openlibrecommunity/olcrtc/internal/handshake"
-	"github.com/openlibrecommunity/olcrtc/internal/link"
 	"github.com/openlibrecommunity/olcrtc/internal/logger"
 	"github.com/openlibrecommunity/olcrtc/internal/muxconn"
 	"github.com/openlibrecommunity/olcrtc/internal/names"
@@ -56,7 +55,7 @@ type HealthFunc func(control.Status)
 
 // Server handles incoming tunnel connections and proxies their traffic.
 type Server struct {
-	ln             link.Link
+	ln             transport.Transport
 	cipher         *crypto.Cipher
 	conn           *muxconn.Conn
 	session        *smux.Session
@@ -89,7 +88,6 @@ type ConnectRequest struct {
 
 // Config holds runtime configuration for [Run].
 type Config struct {
-	Link             string
 	Transport        string
 	Carrier          string
 	RoomURL          string
@@ -240,12 +238,8 @@ func smuxConfig(maxWirePayload ...int) *smux.Config {
 	return cfg
 }
 
-func linkMaxPayload(ln link.Link) int {
-	provider, ok := ln.(link.FeaturesProvider)
-	if !ok {
-		return 0
-	}
-	return provider.Features().MaxPayloadSize
+func linkMaxPayload(tr transport.Transport) int {
+	return tr.Features().MaxPayloadSize
 }
 
 func (s *Server) bringUpLink(
@@ -253,25 +247,24 @@ func (s *Server) bringUpLink(
 	cfg Config,
 	cancel context.CancelFunc,
 ) error {
-	ln, err := link.New(ctx, cfg.Link, link.Config{
-		Transport:        cfg.Transport,
-		Carrier:          cfg.Carrier,
-		RoomURL:          cfg.RoomURL,
-		Engine:           cfg.Engine,
-		URL:              cfg.URL,
-		Token:            cfg.Token,
-		ChannelID:        cfg.ChannelID,
-		DeviceID:         "",
-		Name:             names.Generate(),
-		OnData:           s.onData,
-		DNSServer:        s.dnsServer,
-		ProxyAddr:        s.socksProxyAddr,
-		ProxyPort:        s.socksProxyPort,
-		TransportOptions: cfg.TransportOptions,
-		Traffic:          cfg.Traffic,
+	ln, err := transport.New(ctx, cfg.Transport, transport.Config{
+		Carrier:   cfg.Carrier,
+		RoomURL:   cfg.RoomURL,
+		Engine:    cfg.Engine,
+		URL:       cfg.URL,
+		Token:     cfg.Token,
+		ChannelID: cfg.ChannelID,
+		DeviceID:  "",
+		Name:      names.Generate(),
+		OnData:    s.onData,
+		DNSServer: s.dnsServer,
+		ProxyAddr: s.socksProxyAddr,
+		ProxyPort: s.socksProxyPort,
+		Options:   cfg.TransportOptions,
+		Traffic:   cfg.Traffic,
 	})
 	if err != nil {
-		return fmt.Errorf("failed to create link: %w", err)
+		return fmt.Errorf("failed to create transport: %w", err)
 	}
 	s.ln = ln
 
@@ -287,7 +280,7 @@ func (s *Server) bringUpLink(
 		s.handleReconnect()
 	})
 
-	logger.Infof("Connecting link via %s/%s/%s...", cfg.Link, cfg.Transport, cfg.Carrier)
+	logger.Infof("Connecting transport=%s carrier=%s ...", cfg.Transport, cfg.Carrier)
 	if err := ln.Connect(ctx); err != nil {
 		return fmt.Errorf("failed to connect link: %w", err)
 	}

internal/server/server_test.go

@@ -14,6 +14,7 @@ import (
 	"github.com/openlibrecommunity/olcrtc/internal/control"
 	cryptopkg "github.com/openlibrecommunity/olcrtc/internal/crypto"
 	"github.com/openlibrecommunity/olcrtc/internal/muxconn"
+	"github.com/openlibrecommunity/olcrtc/internal/transport"
 	"github.com/xtaci/smux"
 )
 
@@ -212,14 +213,15 @@ type serverLinkStub struct {
 	closed bool
 }
 
-func (s *serverLinkStub) Connect(context.Context) error   { return nil }
-func (s *serverLinkStub) Send([]byte) error               { return nil }
-func (s *serverLinkStub) Close() error                    { s.closed = true; return nil }
-func (s *serverLinkStub) SetReconnectCallback(func())     {}
-func (s *serverLinkStub) SetShouldReconnect(func() bool)  {}
-func (s *serverLinkStub) SetEndedCallback(func(string))   {}
-func (s *serverLinkStub) WatchConnection(context.Context) {}
-func (s *serverLinkStub) CanSend() bool                   { return true }
+func (s *serverLinkStub) Connect(context.Context) error    { return nil }
+func (s *serverLinkStub) Send([]byte) error                { return nil }
+func (s *serverLinkStub) Close() error                     { s.closed = true; return nil }
+func (s *serverLinkStub) SetReconnectCallback(func())      {}
+func (s *serverLinkStub) SetShouldReconnect(func() bool)   {}
+func (s *serverLinkStub) SetEndedCallback(func(string))    {}
+func (s *serverLinkStub) WatchConnection(context.Context)  {}
+func (s *serverLinkStub) CanSend() bool                    { return true }
+func (s *serverLinkStub) Features() transport.Features     { return transport.Features{} }
 
 func TestShutdownClosesLinkAndConn(t *testing.T) {
 	cipher, err := cryptopkg.NewCipher("01234567890123456789012345678901")