Guard generated platform extensions with V build flags

[antono3]

Summary

Generate platform extension declarations behind V compile-time flags derived from the Vulkan registry's platform/protect metadata. The same flag supplies the matching VK_USE_PLATFORM_* C macro. Extension name and spec-version constants remain available by default.

For example, -d vulkan_xlib enables the Xlib types and commands plus VK_USE_PLATFORM_XLIB_KHR; XCB and Wayland use their own flags. This prevents V3 from emitting C function-pointer typedefs that reference platform types unavailable in the active Vulkan headers.

Verification

• Generated Vulkan bindings locally from Vulkan-Docs v1.3.290 and v1.4.363.
• Compiled the v1.4.363 published binding with released V 0.5.2, both by default and with -d vulkan_xlib.
• Passed pinned V3 frontend checks for default and Xlib opt-in modes.
• The PR CI covers four historical registries, Windows generation, and the pinned V3 frontend.

The checked-in src/vulkan.v snapshot predates the generator's current registry inputs, so it is left unchanged to keep this PR scoped to generator logic. The companion published-binding PR contains the regenerated v1.4.363 output.