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/muxconn/conn.go

@@ -24,16 +24,16 @@ import (
 	"time"
 
 	"github.com/openlibrecommunity/olcrtc/internal/crypto"
-	"github.com/openlibrecommunity/olcrtc/internal/link"
 	"github.com/openlibrecommunity/olcrtc/internal/logger"
+	"github.com/openlibrecommunity/olcrtc/internal/transport"
 )
 
 // ErrClosed is returned from Read/Write after the conn has been closed.
 var ErrClosed = errors.New("muxconn: closed")
 
-// Conn is an io.ReadWriteCloser over a link.Link with optional AEAD wrapping.
+// Conn is an io.ReadWriteCloser over a [transport.Transport] with optional AEAD wrapping.
 type Conn struct {
-	ln     link.Link
+	ln     transport.Transport
 	cipher *crypto.Cipher
 
 	mu     sync.Mutex
@@ -42,9 +42,9 @@ type Conn struct {
 	closed bool
 }
 
-// New wires a Conn over the given link. Push must be set as the link's OnData
-// callback before this conn is used.
-func New(ln link.Link, cipher *crypto.Cipher) *Conn {
+// New wires a Conn over the given transport. Push must be set as the
+// transport's OnData callback before this conn is used.
+func New(ln transport.Transport, cipher *crypto.Cipher) *Conn {
 	c := &Conn{ln: ln, cipher: cipher}
 	c.cond = sync.NewCond(&c.mu)
 	return c

internal/muxconn/conn_test.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	cryptopkg "github.com/openlibrecommunity/olcrtc/internal/crypto"
+	"github.com/openlibrecommunity/olcrtc/internal/transport"
 )
 
 var errMuxBoom = errors.New("boom")
@@ -22,12 +23,13 @@ type stubLink struct {
 	canSendFn func() bool
 }
 
-func (s *stubLink) Connect(context.Context) error   { return nil }
-func (s *stubLink) Close() error                    { return nil }
-func (s *stubLink) SetReconnectCallback(func())     {}
-func (s *stubLink) SetShouldReconnect(func() bool)  {}
-func (s *stubLink) SetEndedCallback(func(string))   {}
-func (s *stubLink) WatchConnection(context.Context) {}
+func (s *stubLink) Connect(context.Context) error    { return nil }
+func (s *stubLink) Close() error                     { return nil }
+func (s *stubLink) SetReconnectCallback(func())      {}
+func (s *stubLink) SetShouldReconnect(func() bool)   {}
+func (s *stubLink) SetEndedCallback(func(string))    {}
+func (s *stubLink) WatchConnection(context.Context)  {}
+func (s *stubLink) Features() transport.Features     { return transport.Features{} }
 func (s *stubLink) Send(data []byte) error {
 	s.mu.Lock()
 	defer s.mu.Unlock()