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/plugin_clock_test.go

@@ -0,0 +1,95 @@
+package lua
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+// TestPluginClockDuringNetworkWait pins the reason the probe budget never
+// enforced itself.
+//
+// Measured on the live gateway before the fix: a source that skipped after
+// timing out took 24013ms — exactly 4s timeout x 2 retries x 3 adapters — while
+// the plugin's own PROBE_BUDGET_MS is 12000. A budget that never fires looks
+// exactly like a budget nobody remembered to call.
+//
+// The cause is os.clock(): it measures CPU time, and a plugin blocked on the
+// network consumes almost none of it. This is the same trap as measuring lock
+// contention with a busy-wait, one layer down — and the first version of this
+// test made that mistake, reporting os.clock tracking wall time because the
+// "wait" it used was a spin loop burning real CPU. It had to be re-measured
+// against an actual HTTP request.
+func TestPluginClockDuringNetworkWait(t *testing.T) {
+	dir := t.TempDir()
+	pluginDir := filepath.Join(dir, "plugins")
+	if err := os.MkdirAll(pluginDir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	vm := NewVM(filepath.Join(dir, "adapters"))
+	if err := vm.Start(); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(vm.Stop)
+	ps := NewPlugins(vm, pluginDir)
+	t.Cleanup(ps.Close)
+	ps.SetOutboundHosts([]string{"127.0.0.1"})
+
+	code := `
+local P = {}
+P.name = "clocktest"
+P.version = "1.0.0"
+P.exports = { "measure" }
+P.measure = function(payload)
+  local c0, w0 = os.clock(), os.time()
+  -- The real thing: block on the network. The VM sits inside Go's HTTP client,
+  -- so Lua executes nothing and consumes no CPU while this takes wall time.
+  __llmsproxy_http({ url = payload.url, method = "GET", timeout_ms = 5000 })
+  return {
+    clock = os.clock() - c0,
+    wall = os.time() - w0,
+  }
+end
+return P
+`
+	if err := os.WriteFile(filepath.Join(pluginDir, "clocktest.lua"), []byte(code), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		time.Sleep(3 * time.Second)
+		_, _ = w.Write([]byte(`{}`))
+	}))
+	defer srv.Close()
+
+	if err := ps.LoadDir(); err != nil {
+		t.Fatal(err)
+	}
+	p := ps.find("clocktest")
+	if p == nil {
+		t.Fatal("plugin not found")
+	}
+	if p.LoadError != "" {
+		t.Fatalf("plugin failed to compile: %s", p.LoadError)
+	}
+	out, err := ps.Call("clocktest", "measure", map[string]interface{}{"url": srv.URL + "/slow"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	m, _ := out.(map[string]interface{})
+	clock, _ := m["clock"].(float64)
+	wall, _ := m["wall"].(float64)
+	t.Logf("3s blocked on HTTP: os.clock advanced %.3fs, os.time advanced %.0fs", clock, wall)
+
+	if wall < 3 {
+		t.Fatalf("the request did not take 3 wall seconds (got %.0f)", wall)
+	}
+	if clock > wall/2 {
+		t.Fatalf("os.clock tracked wall time (%.3f vs %.0f) — the budget is not the cause; "+
+			"look elsewhere", clock, wall)
+	}
+	t.Logf("CONFIRMED: blocked on the network for %.0fs, os.clock advanced %.3fs. "+
+		"A budget written in os.clock never fires while a plugin waits on I/O.", wall, clock)
+}