Support V3 ImGui bindings and pin Vulkan release

[antono3]

Changes

• Pin the stable docking demo to Vulkan v2.0.0 and fixed compiler/dependency commits.
• Clean up generated ImGui/ImPlot C fields and by-value structs and stale ImPlot aliases so the pinned V3 frontend can type check both bindings and the demo.
• Fix the Vulkan bridge's C declarations and explicit mutation; expose GLFW backend C declarations to MSVC.
• Require pinned stable Linux/Windows builds and a pinned V3 frontend check; keep current V/dependency master advisory.
• Pin workflow actions and update the Windows demo revision.

Verification

• Binding cleanup self-test and direct pinned V3 checks passed locally.
• Released V GCC demo compiled after the generated changes; docking UI was visually exercised under Xvfb.
• PR CI covers Linux, Windows, and pinned V3.

Full V3 C backend compilation still encounters raw Vulkan platform-specific extern declarations on Linux. The current-master build reports this in the advisory lane. The related demo source is in antono2/v_imgui_examples#10.