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/core/core.go

@@ -16,7 +16,6 @@ import (
 	"sync/atomic"
 	"time"
 
-	"llmsproxy/internal/billing"
 	"llmsproxy/internal/config"
 	"llmsproxy/internal/lua"
 	"llmsproxy/internal/provider"
@@ -1055,7 +1054,7 @@ func (c *Core) applyBillingDSL() {
 		log.Printf("[billing] no billing profile resolved (active=%q)", dsl.Active)
 		return
 	}
-	prices, err := billing.Compile(profile, c.cfg.Sources)
+	prices, staleRules, err := c.compileBillingPrices(profile, false)
 	if err != nil {
 		// ONE stale rule URL must not take the whole price table down with it.
 		//
@@ -1070,7 +1069,7 @@ func (c *Core) applyBillingDSL() {
 		// So: apply the rules that DO match, and say exactly what was dropped.
 		// The profile still fails loudly — it just no longer fails silently for
 		// everything else.
-		lenient, lerr := billing.CompileOpts(profile, c.cfg.Sources, true)
+		lenient, stale, lerr := c.compileBillingPrices(profile, true)
 		if lerr != nil {
 			log.Printf("[billing] profile %q not applied: %v (lenient retry also failed: %v)",
 				profile.ID, err, lerr)
@@ -1083,34 +1082,21 @@ func (c *Core) applyBillingDSL() {
 		log.Printf("[billing] ERROR profile %q has %d rule URL(s) matching no configured "+
 			"source: %v — applied the remaining %d rule(s); unmatched traffic falls through "+
 			"to default=%q and keeps counting in unpriced_reqs. FIX THE RULE URL: a stale URL "+
-			"used to zero out the entire price table.", profile.ID,
-			len(billing.UnmatchedRuleURLs(profile, c.cfg.Sources)),
-			billing.UnmatchedRuleURLs(profile, c.cfg.Sources),
-			len(profile.Rules)-len(billing.UnmatchedRuleURLs(profile, c.cfg.Sources)), def)
-		prices = lenient
+			"used to zero out the entire price table.", profile.ID, len(stale), stale,
+			len(profile.Rules)-len(stale), def)
+		prices, staleRules = lenient, stale
 	}
+	_ = staleRules
 	if err := ps.SetState("billing", map[string]interface{}{"prices": prices}); err != nil {
 		log.Printf("[billing] applying profile %q: %v", profile.ID, err)
 		return
 	}
 	log.Printf("[billing] profile %q applied (%d rules, %d sources)", profile.ID, len(profile.Rules), len(c.cfg.Sources))
-	// unmatched_sources used to be written by the compiler and read by nobody,
-	// so a profile that silently left half the gateway unpriced looked applied
-	// and healthy. Logging it is the cheap reader: an operator who sees source
-	// names here knows exactly which rules to add (GitCode issue #1 follow-up).
-	if unmatched, ok := prices["unmatched_sources"].([]string); ok && len(unmatched) > 0 {
-		log.Printf("[billing] WARNING %d source(s) match no rule and fall through to the "+
-			"profile default: %v (add rules, or set `default: free` if that is intended)",
-			len(unmatched), unmatched)
-	} else if raw, ok := prices["unmatched_sources"].([]interface{}); ok && len(raw) > 0 {
-		names := make([]string, 0, len(raw))
-		for _, v := range raw {
-			names = append(names, fmt.Sprint(v))
-		}
-		log.Printf("[billing] WARNING %d source(s) match no rule and fall through to the "+
-			"profile default: %v (add rules, or set `default: free` if that is intended)",
-			len(names), names)
-	}
+	// Sources that no rule matched are encoded by the COMPILER (the plugin):
+	// `default: free` writes a {per_request:0} marker, `unpriced` leaves them out
+	// so the plugin's counters see them. The old `unmatched_sources` key was
+	// written by the compiler and read by nobody — deleted rather than kept as
+	// decoration (GitCode issue #1 / PR #2).
 }
 
 // ValidateScopeQuotas checks every scope entry's caps before they are stored.