refactor(edge): Server 协议统一与 wsclient 收敛（Phase 3 Batch 3）

- openflare/agent/relay/flared 协议类型改为 pkg/protocol 别名，删除重复 struct
- 新增 edge/wsclient Preset 表，三组件 wsclient 改为薄包装
- 更新设计文档、计划与 changelog

internal/apps/openflare/relay/protocol_alias.go

@@ -0,0 +1,12 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package relay
+
+import pkgprotocol "github.com/Rain-kl/Wavelet/pkg/protocol"
+
+type ProxyStat = pkgprotocol.RelayProxyStat
+type HeartbeatPayload = pkgprotocol.RelayHeartbeatPayload
+type Config = pkgprotocol.RelayConfig
+type Settings = pkgprotocol.RelaySettings
+type HeartbeatResponse = pkgprotocol.RelayHeartbeatResponse