fix(billing): 源层零价不再覆盖模型价（issue #1）+ default_mode 生效 + 文档/判据补齐

GitCode issue #1：任何按 `mode: token` 且 URL 非通配符的规则定价的源，计费恒为 0，
且 `unpriced_reqs` 也不增长 —— 完全静默。

## 真因
- `compile.go` 给这类规则写源条目 `{prompt:0, completion:0, per_request:0}`，
  本意只是"标记该源已定价"（同源未列入价目表的模型不算 unpriced）；
- `billing.lua` 的 priceFor() 源层在模型层之后、且 `sp.prompt ~= nil` 即覆盖；
- 两者叠加 ⇒ 模型价被清成 0，而 `out.priced` 仍为 true ⇒ 连未定价计数都不涨。

## 修法
1. `compile.go`：token 规则的源条目只写 `{per_request: 0}`（只是标记，不参与 token
   计价）。`free` / `subscription` 保留全零（那里 0 本身就是语义）。
2. `billing.lua`：
   - `priceFor()` 的 `out` 补 `local`（此前是 VM 级全局变量，跨请求残留）；
   - 实现 `profiles[].default` 的 `free|unpriced` 语义：`free` = 刻意免费（不进
     unpriced），`unpriced`（默认）= 未定价债务。此前 compile.go 写 `default_mode`
     但**无人读**，两种取值运行时完全等价。
3. `core.go`：应用 profile 时把 `unmatched_sources` 打进日志（WARNING 列出没有
   规则命中的源）。该字段此前只写不读，是第二个静默少报点。
4. `docs/plugins.md`：token 价优先级改为与实际实现一致的
   `keys > sources > models > default`，并写明"标记已定价只能用 per_request:0"。

## 判据（都先红后绿）
- `internal/billing`：token 规则的源条目**不含** prompt/completion；free/subscription
  仍保留显式 0。
- `internal/lua`：同源同模型下旧形状 cost=0（缺陷本身）、新形状 cost=0.75 且
  unpriced_reqs=0；未列入价目表的模型仍算"已定价"；default_mode 三态。
- `internal/gateway`：`billing_rules_api_test` 的单位断言原本读
  `cc["models"]`（恒 false，整块被跳过——从未执行过），改为 `prices["models"]`，
  并新增"源条目不得带 token 价"断言。

## 顺带修掉一个"永不执行"的判据
`discover_mutate_test.go` 的变异锚点写成 6 个空格，而 `discover.go` 已是 2 个空格
⇒ 该变异每次都 `anchor not found`，变异 campaign 长期红着。修正锚点后，
"嵌套 metadata 不再读取"这条变异真的被执行并被测试 KILL。

internal/billing/compile.go

@@ -245,8 +245,23 @@ func applyRule(prices map[string]interface{}, rule *config.BillingRule, srcName
 		// model priced on its own, so the wildcard loses nothing.
 		if rule.URL != "*" {
 			if _, ok := prices["sources"].(map[string]interface{})[srcName]; !ok {
+				// ONLY the marker — deliberately no "prompt"/"completion".
+				//
+				// priceFor() applies the source dimension AFTER the model
+				// dimension, and treats a present source key as an override
+				// (`sp.prompt ~= nil` => out.prompt = sp.prompt). Writing
+				// zeros here therefore rewrote the model price of every model
+				// this rule DOES list to 0 — while out.priced stayed true, so
+				// unpriced_reqs never grew either. Cost 0, no warning: GitCode
+				// issue #1.
+				//
+				// `per_request: 0` is the marker itself: it adds nothing to a
+				// request's cost, but its presence makes the plugin see the
+				// source as priced, which is the only thing this entry is for.
+				// (free/subscription above keep their zero token prices: there
+				// the zero IS the statement.)
 				prices["sources"].(map[string]interface{})[srcName] = map[string]interface{}{
-					"prompt": 0.0, "completion": 0.0, "per_request": 0.0,
+					"per_request": 0.0,
 				}
 			}
 		}

internal/billing/wildcard_test.go

@@ -212,3 +212,80 @@ func TestLongContextStepCompiles(t *testing.T) {
 		t.Error("above=0 must be rejected")
 	}
 }
+
+// A specific-URL token rule marks its source priced, but the entry must carry
+// ONLY the "this source is priced" marker — not zero token prices.
+//
+// Why: `billing.lua`'s priceFor() applies the source dimension AFTER the model
+// dimension and treats `sp.prompt ~= nil` as an override. A {prompt:0,
+// completion:0} entry therefore zeroes the model price of every model the rule
+// DOES list. The symptom is silent: cost is 0 while out.priced is true, so
+// unpriced_reqs does not grow either (GitCode issue #1).
+//
+// The original test only asserted the KEY existed, which froze the broken shape
+// into a contract; this asserts the entry CONTENT.
+func TestSpecificURLTokenRuleSourceEntryHasNoZeroTokenPrices(t *testing.T) {
+	sources := []config.Source{{Name: "mine", BaseURL: "https://mine.example.com/v1"}}
+	profile := &config.BillingProfile{
+		ID: "p",
+		Rules: []config.BillingRule{{
+			URL: "https://mine.example.com/v1", Mode: "token",
+			Models: map[string]config.BillingToken{
+				"m": {Prompt: "0.15", Completion: "0.60"},
+			},
+		}},
+	}
+	prices, err := Compile(profile, sources)
+	if err != nil {
+		t.Fatalf("compile: %v", err)
+	}
+	entry, ok := prices["sources"].(map[string]interface{})["mine"].(map[string]interface{})
+	if !ok {
+		t.Fatal("a specific-url token rule must still mark its own source priced")
+	}
+	for _, key := range []string{"prompt", "completion"} {
+		if _, present := entry[key]; present {
+			t.Errorf("source entry carries %s=%v: priceFor() treats it as an override "+
+				"and zeroes the model price (issue #1)", key, entry[key])
+		}
+	}
+	// per_request: 0 is the marker itself: adding a flat fee must not change
+	// the total, and the key's presence is what tells the plugin "priced".
+	if v, present := entry["per_request"]; !present || v != 0.0 {
+		t.Errorf("source entry must carry per_request:0 as the priced marker, got %#v", entry)
+	}
+	// The model price must still be the real per-token value.
+	m, ok := prices["models"].(map[string]interface{})["m"].(map[string]interface{})
+	if !ok {
+		t.Fatal("model price missing")
+	}
+	if m["prompt"] != 0.15e-6 {
+		t.Errorf("model prompt price = %v, want 1.5e-7 per token", m["prompt"])
+	}
+}
+
+// free / subscription are DELIBERATE zeros: their source entries keep the
+// zero token prices, because there the zero IS the statement.
+func TestExplicitZeroModesKeepZeroTokenPrices(t *testing.T) {
+	for _, mode := range []string{"free", "subscription"} {
+		profile := &config.BillingProfile{ID: "p", Rules: []config.BillingRule{{
+			URL: "https://mine.example.com/v1", Mode: mode, Subscription: 10,
+		}}}
+		prices, err := Compile(profile, []config.Source{
+			{Name: "mine", BaseURL: "https://mine.example.com/v1"},
+		})
+		if err != nil {
+			t.Fatalf("%s compile: %v", mode, err)
+		}
+		entry, ok := prices["sources"].(map[string]interface{})["mine"].(map[string]interface{})
+		if !ok {
+			t.Fatalf("%s: source entry missing", mode)
+		}
+		for _, key := range []string{"prompt", "completion"} {
+			if v, present := entry[key]; !present || v != 0.0 {
+				t.Errorf("%s: entry[%s] = %v (present=%v), want an explicit 0.0",
+					mode, key, v, present)
+			}
+		}
+	}
+}