## Summary
Stacked on #2676. Review that first; this PR's diff is against it.
Items players sell to NPC vendors go into the vendor's buyback (`ShopBuy`) pack. That pack was purged only when someone opened the buy menu, so a vendor that players sold to but never bought from grew without limit.
- A server operator reported 250k+ items piling up from players crafting and selling on a macro.
- Every one of those items was saved.
- Purging or deleting that vendor then sent one remove packet per item to every nearby client and disconnected them.
The behavior is inherited from RunUO.
- **`VendorBuybackPack`** replaces the plain backpack on `Layer.ShopBuy`.
- It opts in to `SkipsChildSerialization`, so buyback items are never saved.
- Its contents are private like any container's, so removing them reaches only the vendor (no client) and openers. Purges and deletes send nothing to bystanders.
- **Capacity.** The buyback holds at most `250 − buy-list entries` items, which is what the buy gump can show. The oldest items are evicted first. At capacity 0 the sold item is consumed and the seller is still paid.
- **Purge on the restock grid.**
- `Restock()` empties the buyback.
- A purge-only timer, armed on the first sale, covers vendors nobody buys from. It is due at the next restock boundary after now, so a vendor whose restock hasn't run for hours doesn't purge an item the moment it's sold.
- The timer never restocks, so stock doubling and halving is unchanged.
- The old per-item one-hour `LastMoved` check is gone.
- **Existing saves.** A legacy `ShopBuy` pack is replaced on the first tick after load. Thanks to #2676, deleting a pack of 250k+ items takes well under a second.
**Behavior changes for shard operators:**
- A buyback item lasts until the next restock boundary (at most `RestockDelay`), not one hour after its sale.
- A vendor that changes maps drops its buyback.
- A buy window that is open during a purge can show a row whose item is gone. Buying that row does nothing, and reopening shows the current list.
## Test plan
- [x] `VendorBuybackPackTests`: evicts oldest first; capacity 0 consumes the item; purge; a stale front entry can't spin eviction
- [x] `VendorBuybackTests`:
- purge due time stays on the restock grid, including a stale restock and a restock time in the future;
- partial stack sale: the split goes to buyback and the seller keeps the remainder;
- zero capacity consumes the item and still pays;
- restock purges and clears the timer, and the next sale re-arms it;
- deleting the vendor cancels the timer;
- a legacy 20k-item pack is replaced.
- [x] UOContent.Tests and Server.Tests green; schema generator clean
- [x] Manual in-game pass:
- sell and buy back, including a partial stack;
- over-capacity sale;
- purge with no buyer;
- `[restock`;
- vendor facet change;
- restart;
- legacy save with a large `ShopBuy` pack;
- mass selling with bystanders present;
- deleting a vendor with a full buyback.
