Release v0.6.0

• Added independent offscreen buffers for each region, including proper region-local printing, scrolling, and cursor behavior.
• Added region z-order control so regions can be brought to front or sent to back.
• Added support for overlapping regions and partially or fully offscreen regions.
• Made region and terminal resize paths transactional so allocation failures leave the previous state unchanged.
• Added Unicode drawing helpers for horizontal lines, vertical lines, boxes, and shade/block strokes.
• Added box-drawing merge logic so compatible intersections form proper corners, tees, and crosses.
• Added a repo-local Codex skill for GuiTerminal project architecture, workflows, and validation guidance.