fix(lua): InvokeHandler 从不 markDirty —— 无 hooks 的插件状态永不落盘

## 现象
channels 插件重启后账号池静默清空：status 返回 accounts={}、pool=null、log=0 条，
磁盘上 `.channels.state.json` 从未存在过（billing/quota 的 .state.json 都在）。
用户侧表现为「登录了，但重启一次账号就没了」。

## 根因
插件状态持久化有三条分发路径，其中两条会 markDirty：
  - hook 路径（invoke）        → markDirtyLocked ✓
  - 导出函数路径（Call）       → markDirtyLocked ✓
  - HTTP handler 路径（InvokeHandler） → **从不 markDirty** ✗

channels 声明 `hooks = {}`，它的一切状态变更（账号入池、凭据引用、模型黑名单、
事件日志）都只经过自己的 HTTP 路由。于是状态从未被标记为脏，
`.channels.state.json` 从未写出，重启即全部丢失。

billing/quota 侥幸没踩到：billing 有 hooks，quota 走 Call。

## 修复
InvokeHandler 在成功返回前 markDirtyLocked(p)，并在 handler 抛错的早退分支
也标记（抛错前可能已改过状态，那次改动仍然是真实的）。与 Call / hook 路径同模式，
只碰 stateSaver 自己的锁，不重入 p.mu（调用方已持锁）。

## 判据
internal/lua/persist_test.go::TestInvokeHandlerIsPersisted
用「无 hooks、只走 HTTP handler」的插件复现 channels 形态：两次 handler 调用后
Close()（等最后一次 flush），断言 .state.json 存在且含 2 个账号。
变异：撤掉 markDirtyLocked → 测试立即 FAIL（已实测）。

go test ./internal/lua/ 与 ./internal/core/ 全绿。

internal/lua/plugin_http_handler.go

@@ -264,6 +264,10 @@ func (ps *Plugins) InvokeHandler(plugin, fnName string, req *handlerRequest) (*h
 		L.Pop(1)
 	}
 	if errMsg != "" {
+		// A handler that raised partway may already have mutated plugin.state
+		// (an account added before a later step failed), so mark it dirty before
+		// bailing: the mutation it managed to make is still real.
+		ps.markDirtyLocked(p)
 		return nil, fmt.Errorf("plugin %s: %s", plugin, errMsg)
 	}
 	if L.GetTop() < 1 || L.IsNil(-1) {
@@ -273,6 +277,18 @@ func (ps *Plugins) InvokeHandler(plugin, fnName string, req *handlerRequest) (*h
 	if err := luaToJSON(L, -1, &raw); err != nil {
 		return nil, err
 	}
+	// A handler mutates plugin.state just as a hook or an exported Call does, and
+	// this was the one dispatch path that never marked the plugin dirty.
+	//
+	// Consequence (real defect): a plugin with NO hooks — channels is exactly
+	// that, it declares hooks=[] and drives everything through its own HTTP
+	// routes — accumulated accounts, credential refs, model blacklists and its
+	// event log entirely in memory. Nothing ever marked it dirty, so
+	// .channels.state.json was never written and a restart silently wiped the
+	// account pool: the operator saw "logged in", restarted for an upgrade, and
+	// the account was gone. billing/quota escape this only because they declare
+	// hooks and/or use Call, both of which already mark dirty.
+	ps.markDirtyLocked(p)
 	return parseHandlerResponse(raw)
 }