feat(plot): add screen-space overlay helper

[LimiNode]

Summary

• add a minimal PlotOverlay helper for ordinary ImGui widgets over an active ImPlot canvas
• place overlays in screen coordinates with optional plot clipping and configurable offset/padding
• restore the parent ImGui cursor/layout and last-item state after the callback
• add a header-only ImPlot regression covering clipping, cursor preservation, independent instances, Button/Combo and popup interaction

Validation

• MinGW ImGuiX ImPlot build: passed
• CTest: 22/22 passed
• test_plot_overlay_header_only: passed
• clang-format and git diff --check: passed

No existing plot widget behavior was changed.