chore: replace ImageSharp with NetVips in the pathfinder tests

[erwan-joly]

Follow-up to #302, which froze SixLabors at 3.x/2.x because ImageSharp 4 / Drawing 3 gate the Release build on a purchased licence key:

<ValidateLicenseTask ... ContinueOnError="$(Configuration.StartsWith('Debug'))" />

Freezing works but leaves us pinned to a library we can never upgrade. guvusocial-img-processor-svc already moved its image pipeline off ImageSharp to NetVips (3aa38fc), so this follows the same route.

Approach

The drawing here is a grid of axis-aligned cells — filled rects, a few glyphs, arrows. So MapCanvas composites against a plain RGBA byte buffer, and libvips is used only for the two parts that genuinely need a library: glyph rasterisation (Image.Text) and PNG encoding. That also removes SixLabors.Fonts and the system-font probing it needed, since fontconfig resolves sans itself.

Net effect on the tests is that they get shorter — the image.Mutate(ctx => …) wrappers and the five-line RichTextOptions blocks collapse into direct calls:

image.FillRect(x * Scale, y * Scale, Scale, Scale, color);
image.DrawTextCentered(centerX, centerY, text, Colors.Black);

140 insertions, 276 deletions.

Why this is safe

The approval checksums are unchanged. They are a SHA-256 over the listPixel cell colours, not over the rendered image — the PNG is documentation embedded in the approved markdown. So the tests assert exactly what they asserted before, and all 7 pass without touching a single .approved.md.

For that to hold the colour constants had to match ImageSharp's named values byte-for-byte, so I dumped them from ImageSharp 3.1.12 rather than assuming — Color.Green is 008000, not 00FF00, which is the one that would have silently broken the hashes.

Verification

• dotnet test — 7/7 pass, no approved files changed
• dotnet build -c Release — succeeds, no licence gate
• Documentation PNGs regenerated and visually checked: arrowheads, centred labels, the ∞ glyphs and the blue distance gradient all render as before

Native packages are pulled for linux-x64 (CI), win-x64 and osx-arm64.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 014c043 and 8e6640a.

⛔ Files ignored due to path filters (5)

• documentation/brushfire.png is excluded by !**/*.png
• documentation/flow-field-path.png is excluded by !**/*.png
• documentation/flow-field.png is excluded by !**/*.png
• documentation/goal-based-pathfinder.png is excluded by !**/*.png
• documentation/jump-point-search-pathfinder.png is excluded by !**/*.png

📒 Files selected for processing (7)

• test/NosCore.PathFinder.Tests/BrushFireTests.cs
• test/NosCore.PathFinder.Tests/FlowFieldTests.cs
• test/NosCore.PathFinder.Tests/GoalBasedPathfinderTests.cs
• test/NosCore.PathFinder.Tests/JumpPointSearchPathfinderTests.cs
• test/NosCore.PathFinder.Tests/MapCanvas.cs
• test/NosCore.PathFinder.Tests/NosCore.PathFinder.Tests.csproj
• test/NosCore.PathFinder.Tests/TestHelper.cs

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