Add Vulkan backend scaffold

[KonstantinSKY]

What

Add the initial cfg-gated Vulkan backend scaffold under backend-uzu.

Why

This creates a small, compile-checked entry point for follow-up Vulkan backend work without changing existing CPU or Metal behavior.

Validation

• cargo +stable check -p backend-uzu --no-default-features
• cargo +stable check -p backend-uzu --no-default-features --features vulkan
• env CARGO_ENCODED_RUSTFLAGS=-Dwarnings cargo +stable check -p backend-uzu --no-default-features --features vulkan
• cargo metadata --locked --format-version=1
• cargo +nightly fmt --check

[uuuvn]

This "scaffold" serves no purpose in main. We have an old vulkan branch, but our plans regarding other backends have changed to first do webgpu and then reuse it's infrastructure for vulkan later

[KonstantinSKY]

Thanks for clarifying, understood. I won’t open more standalone Vulkan PRs against main.

For the WebGPU-first direction, is the current webgpu branch / #380 still the right public base for external PRs, or are you planning a new WebGPU branch from the current main? If there will be a new base, I can target small follow-up PRs there instead, so I don’t add work to the older dev branch.