Migrate GraphViewer to modern expressions and portable plot geometry

[Olof-Lagerkvist]

GraphViewer previously retained mutable Surface/GDI+ paths and coupled formula evaluation to drawing. This migration uses bound unary expressions, explicit plot definitions and snapshots, and portable geometry from MathExpression 1.1.0 and FunctionPlotting 1.2.0.

• Preserve overlays explicitly across repaint, resize and visibility changes; invalid input retains the last successful graph.
• Use numerical sample differentiation and finite-run trapezoidal integration, with documented gap and initial-value semantics.
• Render screen, print and five image formats through the application's small System.Drawing adapter.
• Remove this application's LTRLib.Windows dependency while retaining all five target frameworks.

Validation: the application owner built and tested the application. The final local package pass compiled all three modern Windows targets. The Windows workflow builds net35/net40 with the full resource toolchain and exercises native drawing, all five encoders, margins, overlays and form resize against the pinned producer packages.

Producer dependency: Library PR #3. Build Library into the shared local NuGet feed first. See docs/graphviewer-migration.md for numerical semantics and manual printer/DPI review notes. FreeBSD work remains deferred.