analysis: split TCP retransmissions into forward and return, and show both on the line cards

tcp.retrans.up counts retransmitted segments sent by the dialing side, tcp.retrans.down those sent by the listening side. The cards show RTT as a round trip (⇄) with forward (→) and return (←) rows for QUIC loss and TCP retransmission.

web/display/src/pages/Overview.tsx

@@ -12,7 +12,7 @@ import { PageError } from '../lib'
 import { IPV6_STATES, IpVersionPicker, ipVersionOf } from '@shared/ipVersions'
 import { LineStrips, OVERVIEW_RANGES, type OverviewRange } from '../LineStrips'
 
-const METRICS = ['rtt.p50', 'rtt.p95', 'rtt.p99', 'loss.total', 'loss.up', 'loss.down', 'tcp.rtt.p50', 'tcp.retrans'].join(',')
+const METRICS = ['rtt.p50', 'rtt.p95', 'rtt.p99', 'loss.total', 'loss.up', 'loss.down', 'tcp.rtt.p50', 'tcp.retrans', 'tcp.retrans.up', 'tcp.retrans.down'].join(',')
 
 type SortKey = 'name' | 'rtt' | 'loss'
 
@@ -136,7 +136,7 @@ function NodeCard({ node, labels, infos }: { node: HostItem; labels: string[]; i
   )
 }
 
-/** 一条线路（拨号方 → 监听方）：近一段时间的 QUIC Datagram 上下行丢包、RTT 和 TCP 重传。 */
+/** 一条线路（拨号方 ⇄ 监听方）：RTT 是往返；去程是拨号方到监听方（左到右），回程是监听方到拨号方（右到左）。QUIC 看两个方向的丢包，TCP 看两个方向的重传。 */
 function LineCard({ l, range, infos }: { l: OverviewLineDto; range: OverviewRange; infos: Map<string, NodeInfoDto> }) {
   const dialerInfo = infos.get(l.dialer_id)
   const agentInfo = infos.get(l.agent_id)
@@ -159,7 +159,7 @@ function LineCard({ l, range, infos }: { l: OverviewLineDto; range: OverviewRang
           {dialerInfo?.country ? <Flag code={dialerInfo.country} /> : null}
           <span className="name">{l.dialer_name}</span>
           {dialerInfo?.isp_key ? <IspIcon ispKey={dialerInfo.isp_key} name={dialerInfo.isp_name} /> : null}
-          <span className="arrow">→</span>
+          <span className="arrow" title="RTT 是往返；去程：左 → 右，回程：右 → 左">⇄</span>
           {agentInfo?.country ? <Flag code={agentInfo.country} /> : null}
           <span className="name">{l.name}</span>
         </span>