Feat: DONUT command (DO) — filled circular ring as LwPolyline

Creates LwPolyline with 2 vertices at (cx ± r_avg, cy), bulge=1.0,
start_width/end_width = (outer - inner) / 2, is_closed = true.

Workflow: inner diameter → outer diameter → click center points (Enter to exit).
Inner diameter 0 → filled solid circle.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/modules/home/draw/mod.rs

@@ -1,5 +1,6 @@
 pub mod arc;
 pub mod circle;
+pub mod donut;
 pub mod ellipse;
 pub mod hatch;
 pub mod line;