feat(distribution): 分站收号压价解耦买家价（方案 B 新口径）

核心语义反转：receive_cost_delta 为压价点数，收号比例 = 主站参考比例 R + 点数，
点数越大号主收号价越低；买家售价锚定主站参考比例推导，不再随收号价联动。

- 迁移 000072：distribution_sites 新增 receive_cost_delta（旧 receive_ratio_delta 保留只读）
- receive_pricing：新增 Normalize/ApplySiteReceiveCostDelta；分站定价买家侧改用
  R 锚点（referenceSellerCoinBasePrice），快照写入 site_receive_cost_delta 与 pricing_model 标记
- site_receive：新增 resolveSiteReceiveCostDelta 读取链路，旧 ResolveReceiveRatioDelta 保留
- 分成不变：压价差经结算层 profit 自动进入 35/40/25 三方分成池；
  货架自主加价仍 100% 归分站，两条边界均以主站为锚
- 权限对齐旧字段：主站全局管理员与分站操作员均可设置
- 前端三端同步：主站 pricing.ts/usePricingCalculator 买卖解耦试算、
  主站后台弹窗改「收号压价」价格语义文案、分销后台 SiteSettings 同步
- 测试：单测锚定 R=43 时 cost_delta=3 → 收号 217.4 / 卖价 263.2 不变 / markup 池 30.6→54.9

frontend/src/stores/distributionSite.ts

@@ -28,6 +28,7 @@ export interface DistributionSiteInfo {
   announcements_json?: string
   footer_info_json?: string
   receive_ratio_delta?: number
+  receive_cost_delta?: number
 }
 
 /**
@@ -74,6 +75,7 @@ export const useDistributionSiteStore = defineStore('distributionSite', {
       announcements: [] as string[],
       footerInfo: null as DistributionFooterInfo | null,
       receiveRatioDelta: 0,
+      receiveCostDelta: 0,
       initialized: false,
     }
   },
@@ -178,6 +180,7 @@ export const useDistributionSiteStore = defineStore('distributionSite', {
           }
 
           this.receiveRatioDelta = Math.max(0, Number(info.receive_ratio_delta || 0))
+          this.receiveCostDelta = Math.max(0, Number(info.receive_cost_delta || 0))
 
           sessionStorage.setItem('hfb_site_code', this.siteCode)
           sessionStorage.setItem('hfb_site_name', this.siteName)
@@ -217,6 +220,7 @@ export const useDistributionSiteStore = defineStore('distributionSite', {
       this.announcements = []
       this.footerInfo = null
       this.receiveRatioDelta = 0
+      this.receiveCostDelta = 0
       sessionStorage.removeItem('hfb_site_code')
       sessionStorage.removeItem('hfb_site_name')
       localStorage.removeItem('hfb_site_code')