Add comprehensive BorderRenderingTests (19/19 passing)

- Border styles: DoubleLine (active), Single, Rounded
- Multiple windows with different border styles
- Border control buttons ([_][+][X])
- Resizable vs non-resizable (resize handle '◢')
- Window overlapping and z-order (BringToFront, SendToBack)
- Fixed: Check border positions instead of content for overlap tests
- Uses ConsoleBufferSnapshot for full-screen coordinates

Author: nickprotop