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/connect.go

@@ -111,9 +111,17 @@ func (h *tunnelHandler) handleConnect(ctx context.Context, req *relay.Request, c
 		af.ParseFrom(dstAddr)
 		resp.Features = append(resp.Features, af) // dst address
 
-		resp.WriteTo(cc)
+		if _, err := resp.WriteTo(cc); err != nil {
+			log.Error(err)
+			cc.Close()
+			return err
+		}
 	} else {
-		req.WriteTo(cc)
+		if _, err := req.WriteTo(cc); err != nil {
+			log.Error(err)
+			cc.Close()
+			return err
+		}
 	}
 
 	t := time.Now()