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.go

@@ -0,0 +1,26 @@
+package tunnel
+
+import (
+	"github.com/go-gost/relay"
+	"github.com/google/uuid"
+)
+
+// parseTunnelID parses a tunnel ID from a string.
+// If s is empty or contains an invalid UUID, the returned tunnel ID is zero
+// (callers must check IsZero). A leading '$' prefix marks the tunnel as private.
+func parseTunnelID(s string) (tid relay.TunnelID) {
+	if s == "" {
+		return
+	}
+	private := false
+	if s[0] == '$' {
+		private = true
+		s = s[1:]
+	}
+	u, _ := uuid.Parse(s) // zero ID on error — caller checks IsZero
+
+	if private {
+		return relay.NewPrivateTunnelID(u[:])
+	}
+	return relay.NewTunnelID(u[:])
+}