feat(pline): commercial solutions keyword parity — Width/Halfwidth/Length and arc sub-modes

PLINE now offers the commercial solutions' full option set: Arc/Close/Halfwidth/Length/
Undo/Width in line mode and Angle/CEnter/CLose/Direction/Halfwidth/Line/
Radius/Second pt/Undo/Width in arc mode, including the nested prompts
(Angle → CEnter/Radius, CEnter → Angle/Length, Radius → Angle → chord
direction). Widths land in the LwPolyline vertices (start/end per segment,
the end width carrying into the next segment) and PLINEWID persists per
session. Undo / Escape inside a sub-prompt only back out of it; every
sub-step previews its arc at the cursor.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/modules/draw/defaults.rs

@@ -21,6 +21,7 @@ thread_local! {
     static RECT_ELEVATION:  Cell<f64> = Cell::new(0.0);
     static RECT_THICKNESS:  Cell<f64> = Cell::new(0.0);
     static RECT_WIDTH:      Cell<f64> = Cell::new(0.0);
+    static PLINE_WIDTH:     Cell<f64> = Cell::new(0.0);   // PLINEWID
     static RECT_ROTATION:   Cell<f64> = Cell::new(0.0);   // degrees
     static RECT_CHAMFER1:   Cell<f64> = Cell::new(0.0);
     static RECT_CHAMFER2:   Cell<f64> = Cell::new(0.0);
@@ -69,6 +70,7 @@ accessors!(get_polygon_sides, set_polygon_sides, POLYGON_SIDES);
 accessors!(get_rect_elevation, set_rect_elevation, RECT_ELEVATION);
 accessors!(get_rect_thickness, set_rect_thickness, RECT_THICKNESS);
 accessors!(get_rect_width, set_rect_width, RECT_WIDTH);
+accessors!(get_pline_width, set_pline_width, PLINE_WIDTH);
 accessors!(get_rect_rotation, set_rect_rotation, RECT_ROTATION);
 accessors!(get_rect_chamfer1, set_rect_chamfer1, RECT_CHAMFER1);
 accessors!(get_rect_chamfer2, set_rect_chamfer2, RECT_CHAMFER2);