GPU context-loss detection compares the ImGui context handle, not the GL context, so ReloadAllTextures never fires #462

What's wrong

CheckAndHandleContextChange() (ImGui.App/ImGuiApp.cs, ~line 2618) is called on every window resize/move (RemapCanvas) and is documented as auto-reloading textures on GL context change. It compares ImGui.GetCurrentContext().Handle against a cached currentGLContextHandle, and calls ReloadAllTextures() when they differ. But ImGui.GetCurrentContext() returns the ImGui context pointer, not the OpenGL context/device — ImGui.CreateContext() is called exactly once, in ImGuiController.Init(), and never again during the app's life. Nothing else in the codebase changes what ImGui.GetCurrentContext() returns (verified ImGuiExtensionManager.cs: ImNodes/ImPlot maintain their own separate contexts and never call ImGui.SetCurrentContext).

Why it matters (failure scenario)

newContextHandle != currentGLContextHandle is never true in real operation, so ReloadAllTextures() is effectively dead code. A real OpenGL context invalidation — a laptop with hybrid/switchable graphics moving the window to a monitor driven by a different GPU, a driver reset, or resuming from sleep on some Linux/NVIDIA setups — recreates the underlying GL context while the ImGui context handle stays identical. Every previously uploaded texture id in the Textures dict now references a deleted/garbage GL texture name on the new context, but the detector never fires, so every image loaded via GetOrLoadTexture renders blank/black/garbage for the rest of the process — silently, with no error and no way to self-heal short of an app restart, contrary to the documented "auto-cleanup on context change" behavior.

Suggested fix / acceptance criteria

Detect the actual GL context instead of the ImGui context — e.g. compare a real context identity Silk.NET exposes, or a GL-side marker set right after gl is (re)created in the window-load handler — rather than ImGui.GetCurrentContext().Handle. Add a regression test that swaps the mock IGL/GL context and asserts ReloadAllTextures fires.