fix(billing/quota): 探测预算用 os.clock 是 CPU 时间，12 秒预算从未生效

## 现象

线上 quota 页面点刷新后卡在「探测中…」，13 秒仍未完成，按钮一直 disabled。

## 实测根因（不是猜测，是量出来的）

单源探测，隔离实例，逐个测：

    juziai   16029 ms
    beizhi   16021 ms
    localzen  2516 ms
    frank     2028 ms
    sensenova  258 ms

而插件自己写着 `PROBE_BUDGET_MS = 12000`。**修复前**这两个源各耗 **24013 ms**
—— 精确等于 4s 超时 × 2 次重试 × 3 个 adapter。预算形同虚设。

## 原因

`exhausted()` 用的是 `os.clock()`，它返回 **CPU 时间**。探测几乎全耗在网络等待上，
CPU 几乎不动，于是预算永远不触发。

判据 `TestPluginClockDuringNetworkWait` 直接量这件事：

    3s blocked on HTTP: os.clock advanced 0.001s, os.time advanced 3s

**001 秒。** 一个用 CPU 时间写的预算，在 I/O 等待下永远不会响。

这个陷阱和「用忙等测锁竞争」是同一个，只是下沉了一层 —— 而且我第一版判据
正是这么写错的：用自旋循环当「网络等待」，结果 os.clock 走了 1.624s / 2s，
看起来 os.clock 跟的是墙钟，差点得出「不是这个原因」的错误结论。必须用真实
HTTP 请求重测才能拿到真相。

代码里两处同样的写法（第 434 和 466 行），一起改掉。

## 改完的效果

    串行 20 源（/call/ 逐个）    84391 ms
    fanout 20 源（并行）          16022 ms
    ⇒ 5.27x

单个死上游从 24s 降到 16s（12s 预算 + 收尾）。

## 还没解决的

16 秒仍然偏长，因为它等于**最慢那一个源**的预算。这不是并行失效，是并行
之后墙钟的必然下界。要再往下走只有两条路：把预算压到 4~6s，或者给已知死掉
的上游走缓存而不是每次重探。都是参数与策略选择，不是结构缺陷，留给下一步。

internal/lua/plugins/quota.lua

@@ -431,10 +431,15 @@ PLUGIN.collect_all = function(payload)
   end
   local srcs = payload.sources
   if type(srcs) ~= "table" then return nil, "missing sources" end
-  local t1 = os.clock()
+  -- os.time, NOT os.clock. os.clock measures CPU time, and a probe spends
+  -- essentially none: measured on this gateway, 3s blocked on an HTTP request
+  -- advanced os.clock by 0.001s. A budget written in os.clock therefore never
+  -- fires, which is why a dead upstream ran the full
+  -- 4s timeout x 2 retries x 3 adapters = 24s against a "12 second" budget.
+  local t1 = os.time()
   local lim1 = PROBE_BUDGET_MS / 1000
   local function spent()
-    return (os.clock() - t1) > lim1
+    return (os.time() - t1) > lim1
   end
   local budget = { exhausted = spent }
   local out = {}
@@ -463,10 +468,10 @@ PLUGIN.collect_source = function(payload)
   -- reads correctly but breaks the moment the call site is `budget.exhausted()`
   -- from inside a pcall-guarded hook where the upvalue chain differs; the
   -- explicit capture below cannot go wrong that way.
-  local t0 = os.clock()
+  local t0 = os.time()
   local limit = PROBE_BUDGET_MS / 1000
   local function exhausted()
-    return (os.clock() - t0) > limit
+    return (os.time() - t0) > limit
   end
   local budget = { exhausted = exhausted }
   for _, s in ipairs(srcs) do
@@ -732,14 +737,21 @@ PLUGIN.ui = {
     var sub=document.getElementById("qa-sub"); if(sub) sub.textContent="探测中…";
     var out;
     try{
-      // One call per source, fired together. Probing is network-bound, so a
-    // sequential collect_all over 21 sources blows past the call's time budget
-    // and every card comes back as one 502 — the probes themselves were fine.
-    // Concurrency also makes the page honest about which sources are slow:
-    // each card appears as its own answer lands.
-    out = await Promise.all(srcs.map(function(s){
-      return pluginAPI.call("quota","collect_source",{sources:[s],name:s.name})
-        .catch(function(e){ return {name:s.name,skipped:true,error:String(e.message||e)}; });
+    // ONE request, fanned out inside the gateway.
+    //
+    // The previous version fired Promise.all at /call/ — twenty concurrent
+    // browser requests. That looked concurrent and was not: each one queued on
+    // the plugin mutex for the whole Lua call, network wait included. Measured on
+    // the live gateway: 29s of wall clock for twenty probes, against 1.4–3.3s for
+    // a single probe with no contention. The page sat on "探测中…" for half a
+    // minute and read as broken.
+    //
+    // The comment that used to sit here claimed concurrency avoided a call budget
+    // overrun. It could not have: the requests were serialised, so the budget they
+    // overran was the very same one. It is a single call now because a
+    // request-per-item fan-out cannot be parallel from the client at all.
+    out = await pluginAPI.fanout("quota","collect_source",srcs.map(function(s){
+      return {sources:[s],name:s.name};
     }));
     // Mirror the host→vendor map the plugin keeps: a source that answered is
     // worth probing first next time, and the page has everything it needs to