Add standard Dear ImGui 1.92.9b variant

[antono3]

Summary

• switch the standard branch from cimgui docking_inter to cimgui master, both based on Dear ImGui 1.92.9b
• regenerate V bindings and vendored sources for the standard upstream variant
• select the matching non-docking Vulkan backend ABI layout
• keep shared ImGui types owned by the ImGui module when generating ImPlot, avoiding duplicate C declarations

Verification

• ./scripts/update_upstream.sh standard
• ./scripts/check_upstream_variant.sh
• no duplicate C struct declarations across imgui.v and implot/implot.v
• native shared library builds successfully
• v_vulkan_video compiles against the standard variant
• GTX 1060 visual/runtime smoke: Big Buck Bunny initializes and plays with the standard build