Update Dear ImGui bindings to 1.92.9b

[antono3]

Summary

• update the default docking bindings to Dear ImGui 1.92.9b, current cimgui, and current cimplot/ImPlot
• regenerate the V bindings and vendored C/C++ sources with current C2V compatibility fixes
• add explicit docking and standard upstream variant tooling while keeping master on docking for compatibility
• switch the hand-written Vulkan backend ABI layout to match the selected upstream variant
• make normal binding generation reproducible without LuaJIT; retain --regenerate-c for upstream C regeneration
• validate the configured variant in CI and enable CI for the future standard branch

Verification

• ./scripts/update_upstream.sh docking
• ./scripts/check_upstream_variant.sh
• generated ImGui and ImPlot bindings contain no duplicate C struct declarations
• native shared library builds successfully on Ubuntu
• v_vulkan_video compiles against the regenerated docking bindings
• GTX 1060 visual/runtime smoke: Big Buck Bunny initializes and plays with the docking build
• standard variant was generated separately and its native library and v_vulkan_video both compile
• GTX 1060 visual/runtime smoke: Big Buck Bunny initializes and plays with the standard build

After this lands, the repository standard branch will be created from the merged result and updated through the same tooling.