refactor(billing)!: 计费全部沉入插件，内核零 billing 代码；规则由插件自持

按维护者要求（issue #1 回复方向）：内核不处理任何 billing 特有逻辑；插件有完整
OS 权限，自己保存配置文件。据此不再保留任何"内核 ↔ billing 插件"的专用对接。

- `internal/billing`（Go 编译器 + 测试）：删除；
- `internal/core` 的 billing 接口/降级应用/规则加载：删除（含 `billingPath`）；
- `internal/gateway` 的 `/api/plugins/billing/rules` 端点与测试：删除；
- `config` 的 `BillingDSL/Profile/Rule/Token/Peak/LongContext` 与校验、
  `billing:`/`billing_file:` 配置段：删除。系统配置里不再有 billing 段。
内核只保留所有插件通用的能力：加载 .lua、插件状态持久化、插件目录
（`__llmsproxy_plugin_dir`）、`plugin.http` 路由、插件可用的 io/os。

- 规则文件移到**插件自己的目录**：`<plugin_dir>/billing.json`，由插件用 io 自己
  读写（tmp + rename 原子替换）；加载插件时自己读文件并编译；
- 规则以**源名** `source` 为键（`url:"*"` 仅表示全局模型价目表）：URL→源名映射
  属于内核配置，插件不为它和内核对接；编辑页在浏览器里取 `/api/sources` 选源名；
- 规则编辑 API 改为插件自己的 `/api/plugins/http/billing/rules`
  （GET/PUT/POST/DELETE，roles=admin），写盘也在插件内完成；
- 计费账目仍在插件状态文件（内核通用持久化）`<plugin_dir>/.billing.state.json`；
- `priceFor` 落地 profile 级 `default` 策略与显式 `mode=unpriced` 的优先级。

生产已迁移：`config.yaml` 的 `billing:` 规则转为 `plugins/billing.json`（按源名
重写），并从系统配置中移除；规则文件与系统配置从此各自独立保存。

- `internal/lua/billing_rules_test.go`：插件自写 billing.json 并重启恢复、
  free/subscription/token/通配符编译语义、1e6+1e6 token = 0.75 端到端、
  非法规则被拒且不覆盖旧文件、reasoning/long_context/peak；
- `internal/lua/billing_test.go`：profile default 策略三态 + 显式 unpriced 优先；
- `internal/lua/billing_ui_test.go`：规则编辑页仍挂在 billing 页内、列宽/列数守卫。

(cherry picked from commit 7941b4110a)

internal/core/core.go

@@ -16,7 +16,6 @@ import (
 	"sync/atomic"
 	"time"
 
-	"llmsproxy/internal/billing"
 	"llmsproxy/internal/config"
 	"llmsproxy/internal/lua"
 	"llmsproxy/internal/provider"
@@ -110,7 +109,6 @@ func NewFromConfig(cfg *config.Config) (*Core, error) {
 		log.Printf("[core] plugin %v %v loaded (hooks=%v ui=%v)",
 			p["name"], p["version"], p["hooks"], p["ui"])
 	}
-	c.applyBillingDSL()
 	c.store = config.NewStore(cfg.RuntimeFile)
 	// Share one box between the runtime store and config.yaml so a single
 	// master.key seals both files. config.Load left the config holding
@@ -1010,63 +1008,6 @@ func (c *Core) Close() {
 	}
 }
 
-// applyBillingDSL compiles the configured billing profile and injects it into
-// the billing plugin.
-//
-// Configured pricing replaces the hand-written JSON blob that used to be PUT
-// through the state API. That path was used twice in production and both times
-// it "succeeded" while being wrong: once without the required `prices` wrapper
-// (silently wiping the accumulated totals), once with a peak-window shape the
-// plugin does not read (peak traffic billed at off-peak rates, no error). A
-// typed config with load-time validation turns both mistakes into startup
-// errors naming the file and field.
-//
-// A DSL with no profile, or one that fails to compile, leaves the plugin's
-// prices untouched: billing keeps running on whatever it had, and the problem
-// is logged rather than becoming a startup failure — the gateway must forward
-// even when its cost accounting is misconfigured.
-func (c *Core) applyBillingDSL() {
-	ps := c.plugins
-	if ps == nil || ps.Count() == 0 {
-		return
-	}
-	// The billing plugin may simply not be installed; a DSL without it is a
-	// no-op, not an error.
-	var have bool
-	for _, row := range ps.List() {
-		if row["name"] == "billing" && row["loaded"] == true {
-			have = true
-			break
-		}
-	}
-	if !have {
-		return
-	}
-	dsl := c.cfg.BillingDSL
-	if dsl == nil || len(dsl.Profiles) == 0 {
-		return
-	}
-	if err := dsl.Validate(); err != nil {
-		log.Printf("[billing] config invalid, profile not applied: %v", err)
-		return
-	}
-	profile := dsl.Resolve(dsl.Active)
-	if profile == nil {
-		log.Printf("[billing] no billing profile resolved (active=%q)", dsl.Active)
-		return
-	}
-	prices, err := billing.Compile(profile, c.cfg.Sources)
-	if err != nil {
-		log.Printf("[billing] profile %q not applied: %v", profile.ID, err)
-		return
-	}
-	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))
-}
-
 // ValidateScopeQuotas checks every scope entry's caps before they are stored.
 // A typo in a period must be rejected at write time rather than silently
 // becoming a never-resetting budget — the opposite of what was typed.