feat(engine): port the viewer engine and panel logic unchanged

Copy the vanilla engine from dwg-dxf-viewer-sample verbatim (#1):

- src/engine/viewer2d/* (Viewer2D.js incl. the 2.5D view, parsers,
  acadrust-dwg WASM, cadSpaces, slugText, linParser/linData,
  fixDwgKoreanText, ...)
- src/engine/{layerPanel,panelUtils,propertyInspector,optionsStore}.ts

The relative layout is kept (engine/viewer2d next to the panel files),
so not even import paths changed; `diff -r --strip-trailing-cr` against
the sample is empty. public/ (NanumGothic, samples, licenses, module
map) is copied as is.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/engine/viewer2d/Viewer2D.d.ts

@@ -0,0 +1,50 @@
+// Types for the ported JS 2D engine (Viewer2D.js). Colocated so tsc resolves
+// `./Viewer2D.js` imports without allowJs. Only the methods the app uses are typed.
+import type { CadSpace } from './cadSpaces';
+
+export class Viewer2D {
+  constructor(container: HTMLElement);
+  /** Render a DWG/DXF parseResult. `keepView` preserves pan/zoom on re-render. */
+  load(result: unknown, opts?: { keepView?: boolean; keepTheme?: boolean; spaceHandle?: string | number | null }): void;
+  /** Zoom-extents to the loaded drawing. */
+  fit(): void;
+  /** Resync renderer/camera to the container size (call after un-hiding). */
+  resize(): void;
+  onSelect(cb: (entity: unknown) => void): void;
+  getSelectedEntity(): unknown;
+  zoomToSelection(): void;
+  setAccent(hex: string): void;
+  setTheme(dark: boolean): void;
+  setGrid(visible: boolean): void;
+  setZoomSpeed(speed: number): void;
+  getZoomSpeed(): number;
+  /** 2.5D (tilted 3D) view: orbit the drawing so entity elevation (Z) is visible. */
+  set25D(on: boolean, opts?: { keepView?: boolean }): void;
+  is25D(): boolean;
+  /** Height exaggeration for 2.5D (1 = true scale); ignored in top view. */
+  setZScale(k: number): void;
+  getZScale(): number;
+  /** 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;
+  /** Distinct lineweights (mm) present in the loaded drawing, ascending. */
+  getLineweightClasses(): number[];
+  getLayerInfo(): { name: string; colorHex: string; count: number; visible: boolean }[];
+  setHiddenLayers(nameSet: Set<string>): void;
+  /** Model / Paper (Layout) spaces for the loaded drawing. */
+  getSpaces(): CadSpace[];
+  getActiveSpace(): string | null;
+  /** Switch Model ↔ Layout; re-renders (fit unless keepView). */
+  setSpace(spaceHandle: string | number | null, opts?: { keepView?: boolean }): void;
+  getZoomPercent(): number;
+  onViewChange(cb: () => void): void;
+  /** Screen client coords → drawing world (Z=0 plane). */
+  screenToWorld(clientX: number, clientY: number): { x: number; y: number; z: number } | null;
+  /** Live CAD coords under the cursor; null when pointer leaves the canvas. */
+  onPointerWorld(cb: ((p: { x: number; y: number; z: number } | null) => void) | null): void;
+  startMeasure(cb: (r: { phase: string; ax: number; ay: number; az: number; bx: number; by: number; bz: number; d: number }) => void): void;
+  stopMeasure(): void;
+}