feat(channel): KOOK 渠道适配器 + 框架兼容增强(wsutil/httpclient)

- plugins/channels/kook: config/adapter/gateway/websocket/send/message/errors — 实现 PlatformAdapter 13方法
  - WS 有状态网关: 取gateway地址→连WS→信令0/1/2/3/4/6 + 30s心跳/6s超时 + resume(拼接sn/session_id) + 指数退避重连
  - 收事件: 信令0 → UnifiedMessage(word注 channel_type/type/target_id/extra.author→字段)
  - 发消息: /message/create(频道) + /direct-message/create(私聊,修正正确端点,非user-chat) + Bearer鉴权
  - 注册 channels.go + CHANNELS_JSON 多账号 + channelbridge.configured 加 kook(bot_token非空才连)
- 框架增强: internal/channel/adapter/wsutil(连接管理/心跳/重连/resume骨架,复用gorilla,KOOK用上) + httpclient(Bearer+envelope归一,KOOK用)
- 全仓 go build/vet/test 绿, 零新依赖(复用餐gorilla), 未动telegram/qq/weixin/webhook契约
- §4.6 隔离/黑白名单/第三方边界未破坏

注: KOOK WS/WH 双模式待补(ReceiveMode), 实测需主人提供 CHANNEL_KOOK_TOKEN

internal/channel/adapter/wsutil/backoff.go

@@ -0,0 +1,54 @@
+// Package wsutil provides a small, reusable WebSocket gateway client for
+// stateful IM channels (KOOK, QQ OneBot, and similar). It owns the transport
+// mechanics that those channels share — dialing a gateway, running a read loop,
+// sending an application-level heartbeat, and reconnecting with exponential
+// backoff (optionally splicing resume parameters onto the gateway URL) — while
+// leaving the platform-specific signaling to the adapter via Handler hooks.
+//
+// It depends only on the standard library plus github.com/gorilla/websocket and
+// internal/logger, never on a specific channel adapter.
+package wsutil
+
+import "time"
+
+// Backoff is a resettable exponential backoff delay scheduler. It yields delays
+// starting at base and doubling up to cap (for example 2s, 4s, 8s, ... capped at
+// 60s), which is the WebSocket gateway reconnect pattern used by stateful IM
+// channels. It is safe for concurrent use by a single goroutine.
+type Backoff struct {
+	base     time.Duration
+	max      time.Duration
+	attempts int
+}
+
+// NewBackoff creates a backoff with the given base and cap. A non-positive base
+// defaults to 1s and a non-positive cap to 60s.
+func NewBackoff(base, cap time.Duration) *Backoff {
+	if base <= 0 {
+		base = time.Second
+	}
+	if cap <= 0 {
+		cap = 60 * time.Second
+	}
+	return &Backoff{base: base, max: cap}
+}
+
+// Next returns the delay to sleep before the next attempt and advances the
+// internal attempt counter. It never exceeds the configured cap.
+func (b *Backoff) Next() time.Duration {
+	var d time.Duration
+	if b.attempts >= 60 {
+		d = b.max
+	} else {
+		d = b.base << uint(b.attempts)
+	}
+	if d > b.max || d <= 0 {
+		d = b.max
+	}
+	b.attempts++
+	return d
+}
+
+// Reset restores the backoff to its initial state after a successful connect, so
+// a later drop starts the ramp over from the base delay.
+func (b *Backoff) Reset() { b.attempts = 0 }