Add a Windows CI workflow that builds and tests with GDI+ and WinForms

[dsn27]

Part of #412: the counterpart of the Linux CI (#461).

Why

The Linux CI compiles some tests out: those that need GDI+, WinForms or OpenGL. They are guarded by WINDOWS, which only the net8.0-windows target of CADability.Tests defines. So far they only ran on a developer's Windows machine:

• the GDI+ conversions of CADability.Forms (Move the GDI+ conversions of the geometry types into CADability.Forms #465);
• the rendering of geometry against reference bitmaps in ProjectTest;
• next, the comparison of the GDI and the managed glyph outlines (step 6, part 3).

What

New workflow .github/workflows/windows-ci.yml. It runs on every pull request, on every push to master, and by hand (workflow_dispatch).

Runner: windows-latest, checkout with submodules: recursive, .NET 8 and 10.

Steps:

• builds the kernel and CADability.Forms;
• runs CADability.Tests (here as net8.0-windows, with CADability.Forms) and CADability.ImportTests;
• uploads the .trx results as an artifact.

The Windows run is a separate workflow, so a failure on Windows does not hide the Linux result and vice versa.

Test fix included: line endings in DxfDimensionTest

The first Windows run showed 4 failing ImportTests:

• import_dxf_dimension_uses_the_standard_style_of_the_file
• import_dxf_degenerate_dimension_keeps_its_block
• import_dxf_dimension_text_override_keeps_the_measurement
• import_dxf_dimension_translates_the_angle_format

Cause: these tests edit their verbatim-string fixtures with Replace("...\n..."). With * text=auto, Windows checks the file out with CRLF, so the search text is not found. The tests then fail on their own "fixture should have been changed" assertion. Linux checkouts have LF, which is why nobody noticed.

Fix: the fixtures are normalized to LF before they are edited (Lf(...)). The file already does the same for the DXF written in another test. This is a test-only change.

Reproduced locally: with the test file converted to CRLF, the 4 tests fail exactly as in CI without the fix, and pass with it. They also pass with LF.

First run on Windows (before the fix)

• CADability.Tests (net8.0-windows): 627 passed, 2 skipped, 0 failed. The OpenGL rendering tests against reference bitmaps pass on the hosted runner.
• CADability.ImportTests: 84 passed, 4 failed (the line-ending issue above).

No change to product code or the csproj version.

🤖 Generated with Claude Code

https://claude.ai/code/session_01YXvDW91oXpy7RquGHNxgR5

[dsn27]

Merged master and added one fix to keep this job green.

RendererRecoveryTests.cs from #497 uses PaintEventArgs with using CADability.Substitutes. The Windows build of the tests sets UseWindowsForms, which also brings System.Windows.Forms.PaintEventArgs into scope, so the test project failed to compile with CS0104. master does not show this because it has no Windows job yet. Merging this PR would have made master red on Windows.

The fix is an alias, using PaintEventArgs = CADability.Substitutes.PaintEventArgs;. I reproduced the error locally with -p:OS=Windows_NT and checked that the build passes with the alias.

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