refactor(billing): 计费语义下沉到 Lua（内核只解 YAML/传数据）+ 采纳 PR #2 语义

按 issue #1 的修复方向（维护者评论）：内核不再持有任何计费语义，billing 逻辑
全部在 `plugins/billing.lua`；内核只负责解 config.yaml、校验 schema、把
profile + 源 base_url 作为**数据**交给插件，再把插件算出的价目通过 `prices`
侧车注入。计费账目仍持久化在插件状态文件（`plugin_dir/.billing.state.json`）。

## 变更
- **删除** `internal/billing`（compile.go 359 行 + 其 Go 测试）；
- 新增 `core.compileBillingPrices`：把 profile/sources JSON 化后调用插件导出
  `compile_prices`，返回价目表 + 未匹配规则清单（供错误日志点名 URL）；
- `applyBillingDSL` / 规则编辑器改为走同一个插件编译入口（编辑器 lenient，
  半成品规则可保存、可报告）；
- `billing.lua` 新增 `compile_prices`：URL 归一匹配、首条命中优先、
  free/subscription/unpriced/token、USD/M→per-token、peak 窗口、
  long_context 边际价、reasoning 可选价、default 策略、未匹配清单。
  错误通过载荷 `error` 字段回传（插件导出第二返回值通道不可靠）。

## 与 PR #2（valuelesser）合并后的语义
- **token 规则不产生任何源条目**：未列价的模型落到 unpriced 统计（可见），
  不再"静默计 $0"。比 issue 正文里的 `{per_request:0}` 标记更收口；
- `default: free` 由**编译期**给未匹配源写 `{per_request:0}` 标记；
  `unpriced`/缺省不写。删除无人读取的 `default_mode` 死键；
- 保留 `unmatched_rules` 清单（只用于降级应用时点名过期 URL）。

## 判据（原 Go 单测平移到 Lua + PR #2 的用例）
- `internal/lua/billing_compile_test.go`：通配符不标记源、token 规则无源条目、
  任意源条目不得携带 token 价（根因直防）、free/subscription 保留有意零价、
  reasoning 缺席、long_context 三种形态、peak 形状、URL 归一+首条优先、
  lenient 未匹配清单、硬错误 5 例、default 两态；
- `internal/lua/billing_test.go`：编译输出驱动真实 priceFor 的 A/B 用例
  （旧形状 cost=0 / 新形状 0.75 且 unpriced=0）、default 编译期语义 e2e；
- `internal/gateway`：token 规则断言"无源条目"、模型价 per-token 量级；
- `internal/core`：过期规则 URL 降级应用（有效规则仍生效）。

internal/gateway/billing_rules_api_test.go

@@ -224,27 +224,34 @@ func TestBillingRulesPersistAndReachThePlugin(t *testing.T) {
 	if !ok {
 		t.Fatalf("prices has no sources map: %#v", prices)
 	}
-	cc, ok := sources["commandcode"]
-	if !ok {
-		t.Fatalf("the rule was saved but never injected: prices has %v, "+
-			"no commandcode entry. Every request on it stays unpriced.", keysOf(sources))
-	}
-	if cc == nil {
-		t.Error("commandcode price entry is nil")
+	// A token rule must NOT produce a source entry at all: priceFor() applies
+	// the source dimension after the model dimension and lets it override the
+	// token rate, so a marker here is what silently zeroed the bill (issue #1 /
+	// PR #2). Models absent from the rule surface as unpriced instead.
+	if entry, ok := sources["commandcode"]; ok {
+		t.Errorf("the token rule produced a source entry %#v — it overrides the "+
+			"model price (issue #1)", entry)
 	}
 	// The free rule must price localzen, and the two must not be conflated.
 	if _, ok := sources["localzen"]; !ok {
 		t.Errorf("the free rule did not reach the plugin: %v", keysOf(sources))
 	}
-	// The source entry exists ONLY as the "this source is priced" marker: it
-	// must not carry token prices, because priceFor() applies the source
-	// dimension after the model dimension and would overwrite the model price
-	// with 0 (GitCode issue #1 — silent, unpriced_reqs does not grow either).
-	if entry, ok := cc.(map[string]interface{}); ok {
+	// Invariant: only DELIBERATE zero-token entries (mode=free/subscription) may
+	// carry prompt/completion, and they must be exactly 0. In this fixture the
+	// free rule covers localzen, so any other source with token prices — or a
+	// non-zero one — would override model rates (issue #1).
+	for name, raw := range sources {
+		entry, ok := raw.(map[string]interface{})
+		if !ok {
+			continue
+		}
+		if name == "localzen" { // the explicit free rule of this fixture
+			continue
+		}
 		for _, key := range []string{"prompt", "completion"} {
 			if _, present := entry[key]; present {
-				t.Errorf("the injected source entry carries %s: it will zero the "+
-					"model price (issue #1): %#v", key, entry)
+				t.Errorf("source %q carries %s=%v: it overrides every model price "+
+					"on that source (issue #1)", name, key, entry[key])
 			}
 		}
 	}