Migrate KAOS project to OpenTK 4 and SDK-style .csproj

[stomppah]

Motivation

• Move the project off the old OpenTK 1.x package and legacy .NET Framework project system to a modern, package-managed OpenTK 4 setup to enable iterative API upgrades.
• Provide a clean, SDK-style baseline so subsequent runtime changes (windowing, input, math, GL modernization) can be implemented incrementally.

Description

• Replaced KAOS/KAOS.csproj with an SDK-style project targeting net8.0-windows and enabled WinForms support while preserving explicit Compile includes for the existing source layout.
• Switched dependency wiring to PackageReference entries and added OpenTK 4.8.2 and OpenTK.GLControl 4.8.2, and updated NUnit and AssimpNet package references.
• Kept the existing binary references to MataliPhysics and MataliRender via HintPath to avoid breaking the current native/binary dependency surface.
• Added KAOS/OpenTK4-MIGRATION.md documenting the migration changes and listing the recommended follow-up runtime refactors (game window/input/math/GL updates).

Testing

• Confirmed the new project file and migration document were added and committed successfully using the repository git commands (commit succeeded).
• No automated build or unit test runs were executed as part of this change; a follow-up should run dotnet build and existing test suites after runtime API updates are applied.

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Codex Task