feat(viewer2d): 2.5D view, viewport fill transform and Z-aware fit box

- 2.5D view: a toolbar toggle (plus an options-panel checkbox) re-aims the
  orthographic camera at an SE-isometric angle and unlocks orbit, so the
  CAD elevation already in the vertex buffers (contours, TIN, 3D
  polylines) becomes visible. Nothing is re-tessellated; switching back
  restores the locked top view. A Z exaggeration slider (persisted in the
  options store) makes kilometre-wide sheets with metres of relief
  legible; picking and the X/Y readout intersect the Z=0 plane.
- Viewport fills: SOLID/HATCH fills and dimension arrowheads drawn
  through a paper-space VIEWPORT now get the model-to-paper transform
  (_fillXf / _bakeFillXf), as lines already did via pushSeg.
- The trimmed content box keeps the drawing's real Z range instead of
  collapsing to z=0, so the 2.5D fit and clip slab cover the relief.
- src/topProgress.ts: driver for the existing #progress loading bar. It
  is not imported from main.ts yet.

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

src/viewer2d/Viewer2D.d.ts

@@ -18,6 +18,12 @@ export class Viewer2D {
   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;