fix(linux): fail early without a display and clarify virtual display setup

[qinghuanandejiangshi]

Problem and solution

On Linux, launching the desktop without a display starts its daemon and then waits indefinitely in GPUI's headless backend. Check gpui::guess_compositor() before calling waku::run() and exit with a graphical-session diagnostic instead. This also handles empty display variables and ZED_HEADLESS; the standalone daemon is unchanged.

The Linux guide now explains the window-manager requirement for bare Xvfb, gives a working Openbox example, and sets both Vulkan driver override spellings for older loaders.

Checks

• Native Linux arm64 executable, Debian 13 container: both integration tests pass (six environment cases). Against the official 0.1.19 executable, the headless regression fails and the configured-display control passes.
• Actual old executable without a display: still running after 5 seconds, with a daemon child. Fixed executable: exit 1 in 7 ms with the display diagnostic.
• Actual X11 windows: old executable on bare Xvfb is black; adding Openbox renders the UI. The fixed debug executable also renders with the documented xvfb-run command. The visual probe uses the official 0.1.19 companion daemon and isolated state, without model calls.
• macOS: formatting, cargo check, full cargo test (975 passed, 39 ignored), protocol consistency, client typecheck and 23 client tests pass. Used gpui_platform/runtime_shaders and the 26.5 SDK because the optional Metal compiler is unavailable locally.
• Checked the old Vulkan 1.3.204 loader source and the current Khronos driver-override documentation.

Limitations and follow-up

No real Wayland compositor was exercised; Wayland configuration is covered by the startup pass-through test. The Linux test build uses Rust 1.98.0 with unoptimized dependencies and debug assertions enabled. Ubuntu 22.04 itself was not run; its documented loader version was checked in upstream source. This does not make bare Xvfb render without a window manager or repair missing graphics drivers.

Related issue

Fixes #236. Fixes #238.

AI disclosure

OpenAI Codex assisted with investigation, implementation, tests, validation and this PR description. The account owner authorized AI-written submission text. The change and recorded results were reviewed; the description is not claimed to be human-written.

Checklist

• The complete change and validation results have been reviewed.
• I wrote this description myself, without LLM-generated text, and will write PR comments myself.
• All AI usage is disclosed, including the tools and extent of involvement.