chestshop_shop data completeness for faithful /find rendering (vanilla item icon + world identity)

Part of PAR-166 (spin out /find). Two small data-fidelity decisions.

1. GUI-icon fidelity for vanilla items. Legacy stores a full serialized ItemStack (Item.item_bytes) for every shop, so its chest GUI always renders the exact icon. Treasury's chestshop_shop.item_data is set only when isCustom (MarketRecords.itemData returns null otherwise); vanilla items carry only material + item_name. For most vanilla items material is enough to build the icon, and isCustom already returns true for anything with ItemMeta (enchanted/anvil-named), so they do get item_data. Verify the new /find GUI can faithfully render every case from material/item_name/item_data; if a gap is found (e.g. plain vanilla with meaningful NBT not flagged custom), widen when item_data is captured.

2. World identity. Legacy keys shops on world UUID (rename-stable, resolvable for unloaded worlds); chestshop_shop uses world name. Decide whether to add a world_uuid BINARY(16) alongside world (low cost, future-proofs world renames / multi-world distance) or accept name as sufficient. Document the decision.

Both are low-priority polish; capture before cutover so the new plugin isn't surprised.