Bundle pinned Vulkan headers and Volk for self-contained builds

[antono3]

Summary

Bundle the pinned Khronos Vulkan C headers and Volk sources with antono2.vulkan so the latest binding compiles against newer generated APIs even when system development headers and Volk are old or absent. Runtime Vulkan loader and driver requirements remain.

• Point the C bridge at the bundled files and make setup.vsh --check validate them.
• Verify bundled sources byte for byte against REGISTRY_COMMIT and VOLK_COMMIT in CI.
• Compile a Vulkan 1.4 API smoke test, including under the strict V3 frontend.
• Document native runtime and platform-window-header requirements.

Local verification

• V 0.5.2 GCC and TCC ergonomic tests: 5/5 each.
• Vulkan 1.4 header/Volk smoke compiled with older system 1.3.275 headers and without VULKAN_SDK.
• Xlib platform smoke compiled.
• v_vulkan_video: 3/3 tests; vulkan_memory_allocator: 7/7 tests against the bundled module.
• Pinned strict V3 frontend accepted the new-header smoke.
• setup.vsh --check passed; workflow YAML parsed; git diff --check passed.

The full V3 C backend remains advisory as in the existing workflow. Cross-platform CI is pending.

v.mod advances to 3.1.2 for a patch release once the PR is merged and CI passes.