feat(infra/domains): 域名池 + CDN 前置 + 签名端点分发 (tsk_NU9JuUweHWMt)

- domains.md: 四组域名隔离登记 + 冷备池 ≥5 + 启用流程（不含身份信息）
- cdn/terraform: Cloudflare 配置即代码（WAF/bot/速率限制/代理DNS/回源鉴权注入）+ 30min 重放 Runbook
- server/internal/originauth: 回源鉴权中间件，非 CDN 网段或鉴权头不符一律 403，支持双值轮换
- tools/endpoint-signer: 离线 Ed25519 签名 CLI（端点 + 公告文档，单调版本防回滚，key_id 双公钥轮换）
- tools/publish-mirrors: ≥3 镜像发布 + hash 一致性校验 + 故障转移取回
- CLIENT-CONTRACT.md: schema/验签/防回滚/合并/兜底链/channel 客户端契约
- 出站独立出口要求写入部署文档；私钥/token/身份信息一律不入库

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infra/domains/examples/mirrors.example.json

@@ -0,0 +1,22 @@
+{
+  "mirrors": [
+    {
+      "name": "cloudflare-pages",
+      "dir": "./_publish/cf-pages",
+      "fetch_url": "https://pages.example-pages.dev",
+      "sync_cmd": ["wrangler", "pages", "deploy", "./_publish/cf-pages", "--project-name", "dist"]
+    },
+    {
+      "name": "github-pages-altacct",
+      "dir": "./_publish/github",
+      "fetch_url": "https://raw.githubusercontent.com/example-alt-account/dist/main",
+      "sync_cmd": ["git", "-C", "./_publish/github", "push"]
+    },
+    {
+      "name": "object-storage",
+      "dir": "./_publish/objstore",
+      "fetch_url": "https://obj.example-storage.com/dist",
+      "sync_cmd": ["aws", "s3", "sync", "./_publish/objstore", "s3://dist", "--profile", "dist-egress"]
+    }
+  ]
+}