docs: record the deferred promotion-window invariant and test-isolation rules

Add the shared promotion-window EXCLUDE-constraint idea to the deferred designs
so the flash/group overlap ordering problem has a recorded structural fix.

Note in AGENTS.md that suite assertions must be scoped to fixtures the test
created, that a list endpoint warrants two consecutive green runs, and that a
piped test command hides the real exit code.

AGENTS.md

@@ -32,6 +32,8 @@ pnpm --filter @vmall/<改动过的前端> build
 ```
 
 - 修 bug：先复现，修完确认复现不再触发；有价值的场景补成集成测试（放 `apps/api/tests/`，复用 `tests/common/mod.rs` 夹具）。
+- 测试隔离：测试库不截断，夹具靠唯一 slug/email。**断言不要假设全局集合为空或恰好 N**——只对自己创建的 id 过滤后断言。新增列表/发现类接口时，`cargo test -p vmall-api` 需**连跑两次**都全绿（单次可能只是库恰好干净）。
+- 管道跑测试时注意退出码：`cargo test | grep` 返回的是 grep 的状态，用 `set -o pipefail` 或显式取 `$?`，否则会漏掉 `FAILED`。
 - UI 变更：起 dev server 用浏览器实际点一遍改动路径，视觉确认才算完成。
 - 不要跑与改动无关的全仓命令（`pnpm -r build`、lint 全仓等）。
 

docs/TBD-marketing.md

@@ -116,6 +116,13 @@ transaction rules before implementation.
 - [ ] **Withdrawals / 提现** — model a payout request with account, amount minor, fee minor,
   destination snapshot, and `requested → approved | rejected | paid` transitions. Funds must
   be frozen and released or debited atomically with the transition.
+- [ ] **Promotion window exclusivity as a database invariant** — flash sales and group buying
+  currently keep one SKU out of overlapping activity windows with cross-table queries, and the
+  flash-sales guard stays inert until the group-buying table exists. A shared
+  `promotion_windows(sku_id, kind, starts_at, ends_at)` table with
+  `EXCLUDE USING gist (sku_id WITH =, tstzrange(starts_at, ends_at) WITH &&)` would make an
+  overlap impossible to insert and remove the creation-order problem. Deferred because it
+  retrofits the archived flash-sales schema; revisit if a third timed activity type appears.
 
 ## Deliberately out of scope — does not block deleting this file