Add ImNodes, ImPlot, and ImGuizmo extension support with centralized management

[Copilot]

Integrates three Hexa.NET.ImGui extensions (ImNodes, ImPlot, ImGuizmo) with proper context management and comprehensive demo implementations.

Core Library

• ImGuiExtensionManager: Centralized extension lifecycle management with automatic context setup/teardown, frame handling, and OpenGL context change detection
• Context Management: Per-frame context setup in ImGuiController ensures extensions operate in correct ImGui context
• Window State Tracking: Enhanced visibility and focus tracking for improved window management

Demo Application

• Modular Architecture: Refactored monolithic demo into 13 separate IDemoTab implementations
• ImNodesDemo: Node editor with physics-based auto-layout using force-directed graph positioning
• ImGuizmoDemo: 3D gizmo manipulation (translate, rotate, scale) with camera controls
• ImPlotDemo: Real-time plotting with multiple chart types

Dependencies

<PackageVersion Include="Hexa.NET.ImGuizmo" Version="2.2.8.4" />
<PackageVersion Include="Hexa.NET.ImNodes" Version="2.2.8.4" />
<PackageVersion Include="Hexa.NET.ImPlot" Version="2.2.8.4" />

Extensions are initialized through ImGuiExtensionManager which handles context binding automatically:

extensionManager.AddExtension("ImNodes", ImNodesNative.CreateContext, ImNodesNative.DestroyContext, ...);
extensionManager.SetupContexts(ImGui.GetCurrentContext());
extensionManager.NewFrame();

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