feat: enable Linux Bevy UI lab for lavapipe smoke

[jeffglousher]

Summary

• Compile the offscreen bevy-ui-lab HUD (dev-ui-lab) on Linux so Mesa llvmpipe (CPU Vulkan) can capture the production Bevy HUD without a GPU.
• Proven on Debian with WGPU_BACKEND=vulkan + VK_ICD_FILENAMES=…/lvp_icd.json: adapter llvmpipe (LLVM 19.1.7), 1440×900 PNG.
• Lab/smoke only. This is not the product GTK/WebKit sibling embed. Linux NativeViewport remains a stub.

Refs #48 (Linux rendering / embed) and #37 (viewport compositor smoke; Linux was out of scope there).

What this is not

Product Linux viewport still needs the GTK/WebKit embed described on #48. This PR only lets the existing offscreen HUD lab compile and run on Linux.

Test plan

• cargo build --bin bevy-ui-lab --features dev-ui-lab on Debian (no GPU)
• cargo run --bin bevy-ui-lab --features dev-ui-lab -- /workspace/bevy-ui-linux-sw.png with lavapipe; adapter is CPU Vulkan; PNG is 1440×900 and non-empty
• macOS/Windows dev-ui-lab still compiles (cfg additive only)

@jackControls please review; do not merge until you sign off.

[jackControls]

Thanks Jeff — the lavapipe proof is useful. I found one blocker before merging:

The Linux Bevy dependency enables x11, which also enables bevy_winit for every Linux build, even without dev-ui-lab. That makes the offscreen lab depend on an X event loop and conflicts with PR #57’s GTK-embedded native viewport, where Bevy must not own the window/event loop.

Could you remove the x11 feature and add a Linux CI smoke test that builds/runs bevy-ui-lab with lavapipe and verifies the 1440×900 PNG? Ideally run it with DISPLAY unset to preserve truly offscreen behavior.

PR #57 already includes the Linux module/font setup, and the branches currently conflict in Cargo.toml and ui.rs. The cleanest route may be to fold the lavapipe regression test into #57 and close #56 as superseded, or rebase #56 onto #57 and retain only that test.

[jeffglousher]

Jack's #57 is the product Linux embed and overlaps the Cargo/mod/ui cfg here. I'll shrink this to the lavapipe dev:bevy-ui:capture script after #57 merges — no need to land both as-is.

[jeffglousher]

Closing as superseded by #57 (now merged). Jack was right: the Linux x11 Bevy feature would pull bevy_winit into the product GTK embed. Lavapipe stays a CI fallback on #57; no separate lab PR.