补齐游戏分发并发上传与幂等重放用例并修掉并发落败的错误码

- 新增 scripts/check-game-distribution-upload-concurrency.mjs：同 key 重放建游戏/建版本返回同一条记录、换请求体 409、同版本同偏移并发写片只成功一片、权威已收字节等于单份包长、确认校验通过落到 uploaded、换 key 重复送审 409 且版本数仍为 1
- 分片追加失败后回读权威已收字节：长度已前进时返回 409 PACKAGE_UPLOAD_OFFSET_MISMATCH 与 receivedBytes，不再把 OSS position 冲突报成 502 UPSTREAM_ERROR
- package.json 注册 check:game-distribution-upload-concurrency
- 游戏分发里程碑阶段 A 第 4 条改为已勾选，并把第 5 条的缺口收窄到「响应丢失与进程退出后的续传编排」

package.json

@@ -75,6 +75,7 @@
     "check:game-distribution-dto-parity": "node scripts/check-game-distribution-dto-parity.mjs",
     "check:game-distribution-media-e2e": "node scripts/check-game-distribution-media-e2e.mjs",
     "check:game-distribution-owner-isolation": "node scripts/check-game-distribution-owner-isolation.mjs",
+    "check:game-distribution-upload-concurrency": "node scripts/check-game-distribution-upload-concurrency.mjs",
     "check:production-ops": "node scripts/check-production-ops-guardrails.mjs",
     "check:preview-deployer": "node scripts/check-preview-deployer.mjs",
     "check:maintenance-page": "node scripts/check-maintenance-page.mjs",