Introduce modern math expressions, plotting and SkiaSharp rendering

[Olof-Lagerkvist]

The historical expression and graph APIs combined parsing, evaluation, sampling and drawing, which prevented portable server-side PNG rendering and made consumer behavior difficult to maintain. This branch implements the accepted design and the consumer-tested rendering and numerical calculus slices.

• Add immutable mathematical syntax, diagnostics, binding, interpreted evaluation and portable expression-tree layout.
• Add parser-independent sampling, clipped plot geometry, numerical differentiation and finite-run integration.
• Keep SkiaSharp rendering and PNG encoding at the edge; legacy APIs remain available for other consumers.
• Prepare stable packages: MathExpression 1.1.0, FunctionPlotting 1.2.0 and Graphics.SkiaSharp 1.1.0.
• Use ArgumentNullException.ThrowIfNull on supported frameworks, with preprocessor fallbacks; use the corresponding supported negative-range guard for source spans.

Validation: the application owner built and tested all consumers successfully. The final local pass built the portable packages across all nine target frameworks and the renderer across .NET 8/9/10. Packages were then consumed through a shared NuGet feed by netexpr, Windows GraphViewer and both full-site targets; CLI scenarios and HTTP/native PNG checks passed. Repository CI passed on Windows, Linux and macOS. Windows passed the main suite on .NET Framework 4.8 and .NET 8/9/10, plus all four renderer tests on each modern runtime; the platform-specific skip remains intentional. Renderer tests now have their own output directory so their test-host version cannot collide with the main test project; CI tests the already-built assemblies.

Review the language and numerical semantics documented in docs/math-expression-redesign, and the final release-checkpoint.md. Merge/build this producer before the companion MathTools, WindowsTools and website migrations. The ltrwebdb serializer fix is a separate PR and precedes the website's database package update. FreeBSD compatibility remains deferred. No packages have been published to NuGet servers.

Companion PRs: MathTools #1, WindowsTools #1, ltrwebdb #1, ltr-data.se #1.