Expose runtime ImGui variant helpers

[antono3]

Summary

• expose the selected upstream variant and docking/viewports capability constants
• add safe helpers for enabling docking, creating the main dockspace, and rendering platform viewports
• make the standard implementation contract explicit: unsupported configuration returns false and rendering helpers are no-ops
• verify the helper/header pairing in the upstream-variant check

Verification

• ./scripts/check_upstream_variant.sh
• compiled the V GLFW/Vulkan demo against the docking module with the CI-pinned V compiler
• git diff --check