- Add attachItemFromGrid server action for drag-drop attachment
- Add attach drop detection in LockerGrid handleDragEnd
- Add attach drop styling with amber highlight on weapon footprint
- Add isAttachDrop prop to GridCell for attach-specific feedback
- Add dragError state with 5s auto-clear for attach failures
- Reuse setItemAttachment/removeItemAttachment semantics
- Handle slot replacement with no-room fallback for old attachments