feat: hide cursor when terminal is unfocused

[arvsrn]

• Hides the canvas cursor and stops blinking while the terminal or desktop window is unfocused
• Makes open() mount-only so cached terminals cannot steal newer focus
• Updates focus() and blur() to target the hidden textarea
• Adds focused regression coverage and updates demos to call focus() explicitly
• Keeps existing method signatures and adds no new public API
• Includes generated dist artifacts in a separate commit