Validate DrawLine precision fix for large bitmaps

[Copilot]

PR #116 fixed a 39-pixel precision error in DrawLine when drawing on large bitmaps (e.g., 30000×10000). This PR validates the fix works correctly.

Validation Results

DrawLine - 8-bit → 24-bit fixed-point precision:

• Before: 39-pixel error on (0,0)→(29999,9999)
• After: 0-1 pixel error (inherent to integer division)
• Status: ✅ Fixed

DrawLineAa - 1-pixel endpoint offset:

• Intentional behavior, not a bug
• Anti-aliasing requires 1-pixel border for blending
• Endpoints clamped to [1, width-2] range
• Status: ✅ Working as designed

Test Coverage

Validated multiple line types on large bitmaps:

• Diagonal: (0,0)→(49999,49999) - 0 pixel error
• Steep: (0,0)→(1000,9999) - 0 pixel error
• Shallow: (0,0)→(29999,1000) - 0-1 pixel error
• Negative slope: (0,9999)→(29999,0) - 0 pixel error

Documentation

• VALIDATION_REPORT.md: Technical analysis of the fix
• ISSUE_VALIDATION_SUMMARY.md: User-facing summary

Original prompt

---

This section details on the original issue you should resolve

<issue_title>DrawLine and DrawLineAa don't draw correctly on larger WriteableBitmaps</issue_title>
<issue_description>Drawing a line over large distances with DrawLine, the end will be off. In the example below there is a difference of 39 pixels in height.
Drawing a line with DrawLineAa, the end will be of by 1 in width and height.

 WriteableBitmap image = new WriteableBitmap(30000, 10000, 96, 96, PixelFormats.Pbgra32, null);
 image.Clear(Colors.White);
 image.DrawLine(0, 0, 29999, 9999, Colors.Black);
 image.DrawLineAa(0, 0, 29999, 9999, Colors.Red);

</issue_description>

<agent_instructions>Check the latest comments and verify if it's a bug or by design. Only address if it's really a bug and you can avoid any regression. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

@reneschulte Hi @lsilvand see the PR that was created. Looks OK to me but please try the PR and see if it works for you all.

• Fixes DrawLine and DrawLineAa don't draw correctly on larger WriteableBitmaps #115

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