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/dxfHatchHandler.d.ts

@@ -0,0 +1,5 @@
+/** HATCH entity handler for dxf-parser's registerEntityHandler(). */
+export default class HatchHandler {
+  ForEntityName: string;
+  parseEntity(scanner: unknown, curr: { code: number; value: unknown }): unknown;
+}