refactor(handler/tunnel): split 794-line entrypoint.go into 6 files, fix 3 bugs, add 48 tests

Breaking Change: moves Connector, ConnectorPool, parseTunnelID from
tunnel.go into dedicated connector.go and id.go.

Bug Fixes:
- dialer.go: clear stale retry err so SD fallback is not masked (nil err
  from pool.Get() left previous iteration's error live)
- connect.go: check WriteTo errors in both mux and non-mux paths
- eprelay.go: use fresh relay.Response for features sent over mux
  (previously reused same resp struct after resp.WriteTo(muxConn))

Refactor:
- entrypoint.go: split ~790 lines into 6 files (ephttp, eptls, eprelay,
  epwebsocket, eplistener, entrypointsvc), moved to dedicated package
- handler.go: moved initEntrypoints/createEntrypointService to
  entrypointsvc.go
- tunnel.go: moved Connector/ConnectorPool/parseTunnelID to
  connector.go and id.go
- connect.go: moved entrypoint-originated handleConnect to eprelay.go
  (handler-side handleConnect kept in connect.go)

Tests (48 new, 88 total):
- id_test.go: parseTunnelID variants (empty, UUID, private '$', invalid)
- tunnel_test.go: Tunnel lifecycle (AddConnector, GetConnector
  weighted/closed filtering, CloseOnIdle, clean),
  Connector (NewConnector nil-opts, GetConn nil/closed session, Close,
  IsClosed), ConnectorPool (Add/Get/Close, idle cleanup, concurrency)
- dialer_test.go: SD error paths (sd.Get error, empty address)
- handler_test.go: observeStats (nil observer, cancelled context loop),
  initEntrypoints (no entrypoints configured)
- helpers_test.go: testLogger, fakeConn for handler tests
- eprelay_test.go: handleConnect ingress/dial/SD round-trip

handler/tunnel/id_test.go

@@ -0,0 +1,97 @@
+package tunnel
+
+import (
+	"testing"
+
+	"github.com/go-gost/relay"
+	"github.com/google/uuid"
+)
+
+func TestParseTunnelID(t *testing.T) {
+	t.Run("empty string", func(t *testing.T) {
+		tid := parseTunnelID("")
+		if !tid.IsZero() {
+			t.Error("expected zero tunnel ID for empty string")
+		}
+	})
+
+	t.Run("valid uuid", func(t *testing.T) {
+		tid := parseTunnelID("6ba7b810-9dad-11d1-80b4-00c04fd430c8")
+		if tid.IsZero() {
+			t.Error("expected non-zero tunnel ID")
+		}
+		if tid.IsPrivate() {
+			t.Error("expected non-private tunnel ID")
+		}
+	})
+
+	t.Run("valid uuid with $ prefix", func(t *testing.T) {
+		tid := parseTunnelID("$6ba7b810-9dad-11d1-80b4-00c04fd430c8")
+		if tid.IsZero() {
+			t.Error("expected non-zero tunnel ID")
+		}
+		if !tid.IsPrivate() {
+			t.Error("expected private tunnel ID")
+		}
+	})
+
+	t.Run("invalid uuid", func(t *testing.T) {
+		tid := parseTunnelID("not-a-uuid")
+		if !tid.IsZero() {
+			t.Error("expected zero tunnel ID for invalid input")
+		}
+	})
+
+	t.Run("invalid uuid with $ prefix", func(t *testing.T) {
+		tid := parseTunnelID("$not-a-uuid")
+		if !tid.IsZero() {
+			t.Error("expected zero tunnel ID for invalid input")
+		}
+	})
+}
+
+func TestParseTunnelID_PrivateMarker(t *testing.T) {
+	t.Run("dollar not at start", func(t *testing.T) {
+		tid := parseTunnelID("6ba7b810-9dad-11d1-80b4-00c04fd430c8")
+		if tid.IsPrivate() {
+			t.Error("expected non-private when $ is not at start")
+		}
+	})
+}
+
+func TestParseTunnelID_RoundTrip(t *testing.T) {
+	original := parseTunnelID("6ba7b810-9dad-11d1-80b4-00c04fd430c8")
+	t.Logf("original string: %s", original.String())
+
+	reparsed := parseTunnelID(original.String())
+	if !original.Equal(reparsed) {
+		t.Errorf("round-trip failed: original=%v reparsed=%v", original, reparsed)
+	}
+}
+
+func TestParseTunnelID_PrivateRoundTrip(t *testing.T) {
+	original := parseTunnelID("$6ba7b810-9dad-11d1-80b4-00c04fd430c8")
+	if !original.IsPrivate() {
+		t.Fatal("expected private tunnel ID")
+	}
+	t.Logf("original string: %s", original.String())
+
+	// The private flag is carried in the struct, not in the string representation.
+	// Reparsing from String() gives a non-private ID because the $ prefix is not
+	// part of the relay.TunnelID.String() output.
+	reparsed := parseTunnelID(original.String())
+	if !original.Equal(reparsed) {
+		t.Errorf("round-trip failed: original=%v reparsed=%v", original, reparsed)
+	}
+}
+
+func TestParseTunnelID_RelayCompatibility(t *testing.T) {
+	id := parseTunnelID("6ba7b810-9dad-11d1-80b4-00c04fd430c8")
+	u, _ := uuid.Parse("6ba7b810-9dad-11d1-80b4-00c04fd430c8")
+	var raw [16]byte
+	copy(raw[:], u[:])
+	expected := relay.NewTunnelID(raw[:])
+	if !id.Equal(expected) {
+		t.Error("parseTunnelID result should match relay.NewTunnelID")
+	}
+}