Make Vulkan setup one command across platforms

[antono3]

Summary

• replace the download-only helper with install and read-only check modes for Linux, macOS, and Windows
• install through apt, dnf, pacman, zypper, the official LunarG macOS installer, or winget
• verify V/C tools, Vulkan and Volk headers, and loader/device availability with actionable driver guidance
• accept both common Volk header layouts and exercise setup diagnostics in the supported OS matrix

Local verification

• v -check-syntax setup.vsh
• Windows and macOS compile-time branches pass V syntax checking
• v run setup.vsh --check
• v test ergonomic (4/4 passed)
• workflow YAML parse and git diff --check