fix(viewer2d): show lineweight at AutoCAD's display scale (20 px/mm)

Hand test against an AutoCAD capture: the 0.50mm road edge is ~10px
there but only ~4px here. Lineweight is zoom-independent screen pixels,
so the two captures compare directly - the 8 px/mm mapping was 2.5x too
small. AutoCAD's default display scale is one pixel per 0.05mm, so use
20 px/mm (0.35mm = 7px, 0.50mm = 10px, 2.11mm = 42px) and raise the
clamp to 48px.

Add setLineweightScale()/getLineweightScale() so a host can retune it -
AutoCAD exposes the same thing as the Adjust Display Scale slider.

Re-measured headless: the 0.35mm road-gutter line now renders ~7px, so
both weight classes land on the same scale.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/viewer2d/Viewer2D.d.ts

@@ -19,6 +19,9 @@ export class Viewer2D {
   /** Lineweight display (AutoCAD LWDISPLAY); null follows the drawing's own flag. */
   setLineweightEnabled(on: boolean | null): void;
   getLineweightEnabled(): boolean;
+  /** Lineweight display scale in px per mm (AutoCAD's display-scale slider). Default 20. */
+  setLineweightScale(pxPerMm: number | null): void;
+  getLineweightScale(): number;
   getLayerInfo(): { name: string; colorHex: string; count: number; visible: boolean }[];
   setHiddenLayers(nameSet: Set<string>): void;
   /** Model / Paper (Layout) spaces for the loaded drawing. */