Advanced layout viewport functionality, including non-rectangular viewports, clipping, and comprehensive MVIEW/VPCLIP commands.

## Summary
Layout viewports are limited to rectangles (`docs/gaps.md` F15). Missing:
- polygonal, object and clipped viewports;
- MVIEW's Lock, ON/OFF, Fit, Restore and 2/3/4 options;
- VPCLIP;
- a viewport scale control in the UI.

## Scope
- MVIEW: Polygonal (pick vertices, with Arc segments where it fits), Object (convert a closed polyline, circle, ellipse, spline or region into a viewport boundary), Lock On/Off, ON/OFF, Fit, Restore (from named viewport configurations if VPORTS exist; otherwise "not available yet"), 2/3/4 viewports, with both the typed form and the JSON form.
- VPCLIP: clip an existing viewport with an object or a polygon, and Delete to remove the clip.
- Clipped viewports: render model space clipped to the boundary on screen and in PNG/SVG/PDF plots. A locked viewport refuses zoom and pan inside it.
- DXF: viewport clip boundary handle (group 340), status flags (90: locked, non-rectangular clipping, on/off), round trip per the public DXF Reference.
- UI: a viewport scale list on the status bar for the active viewport, kept thin.
- Programmatic calls never open dialogs.

## Expected
Viewports behave like publicly documented AutoCAD layout viewports.