Add configurable image padding to core API and CLI

[Copilot]

This adds explicit padding control to image exports so renders can reserve whitespace around the drawing. The CLI now accepts a single padding value, x/y padding, or per-side padding.

• Core configuration

  • Adds per-side padding to ImageConfiguration:
    • PaddingLeft
    • PaddingTop
    • PaddingRight
    • PaddingBottom
  • Adds SetPadding(...) overloads for:
    • uniform padding
    • horizontal/vertical padding
    • per-side padding

• Rendering behavior

  • Applies padding inside the configured output dimensions rather than increasing the final canvas size
  • Computes scale and offsets from the remaining drawable area
  • Rejects padding values that fully consume the drawable width or height

• CLI support

  • Adds --padding / -p
  • Supports these forms:
    • --padding 10
    • --padding 10,20
    • --padding 10,20,30,40
  • Maps values as:
    • 10 → all sides
    • 10,20 → left/right=10, top/bottom=20
    • 10,20,30,40 → left,top,right,bottom

• Docs and examples

  • Updates CLI help text and README usage examples to document the new option and value formats

• Targeted coverage

  • Adds focused tests for default padding state, page-context padding math, and invalid padding that leaves no drawable area

Example:

cad-to-image drawing.dxf --width 1600 --height 900 --padding 24
cad-to-image drawing.dxf --width 1600 --height 900 --padding 24,12
cad-to-image drawing.dxf --width 1600 --height 900 --padding 24,12,40,20

var exporter = new ImageExporter();
exporter.Configuration.Width = 1600;
exporter.Configuration.Height = 900;
exporter.Configuration.SetPadding(24, 12, 40, 20);