fix(steami_screen): Improve gauge reactivity and readability.

[MatteoCnda1]

Summary

Improve gauge_demo.py reactivity and readability on hardware. Closes #370

Changes

• Added arc_width optional parameter to gauge() in device.py
  • Defaults to max(5, radius // 9) if not provided — no breaking change
• Reduced _draw_arc() oversampling from x2 to x1.5 for faster rendering
• Updated gauge_demo.py:
  • Dynamic arc color based on distance zones (WHITE / YELLOW / LIGHT)
  • Dynamic arc width — thicker when close, thinner when far
  • Moving average smoothing over 5 samples to avoid abrupt jumps
  • Redraw only when distance changes by more than 5mm (REDRAW_THRESHOLD)
  • Replaced screen.value() with screen.text() at scale=1 for readability
  • Added try/finally to clear display on exit

Checklist

• ruff check passes
• python -m pytest tests/ -k mock -v passes
• Tested on hardware (STM32WB55 / STeaMi board)
• Examples added/updated (lib/steami_screen/examples/gauge_demo.py)
• Commit messages follow <scope>: <Description.> format