feat(linux): support Ubuntu 26.04 desktop

[jackControls]

Summary

• add a native Bevy/wgpu Vulkan viewport for Linux using an embedded GTK/X11 child, including Ubuntu Wayland sessions through XWayland
• add Ubuntu 26.04 .deb and AppImage packaging, dependency auditing, X11/XWayland viewport probes, and CI artifacts
• document Linux packaging and OCCT requirements while keeping the existing macOS and Windows renderer behavior behind their current platform paths
• harden first-size negotiation, resize/full-screen transitions, workspace suspension, overlay cutouts, and GTK input handling

Validation

• npm run build:desktop
• cargo check --manifest-path src-tauri/Cargo.toml
• cargo test --manifest-path src-tauri/Cargo.toml --lib — 30 passed
• exact Ubuntu 26.04 release build produced and audited both .deb and AppImage packages
• packaged viewport verified under X11 and XWayland
• physical Ubuntu 24.04 NVIDIA host smoke-tested through jackgpu: viewport rendering, menu/dialog layering, workspace switching, resizing, and basic UI clicks

WIP / merge gates

• require green Linux, macOS, and Windows package jobs because shared viewport/build files changed
• review the complementary Linux Bevy UI-lab work in feat: enable Linux Bevy UI lab for lavapipe smoke #56 for overlap before merge

The implementation is platform-gated: Linux uses GTK/X11/Vulkan-specific lifecycle settings, while macOS keeps AppKit/Metal and Windows keeps HWND/DX12-Vulkan behavior.

[jeffglousher]

This is the real Linux viewport. GTK/X11 child under WebKit, XWayland for Ubuntu Wayland, Ubuntu 26.04 .deb/AppImage already green, and the 24.04 NVIDIA smoke is the right bar.

#56 is only the offscreen lavapipe HUD lab. It overlaps Cargo/mod/ui cfg; I'll shrink it after this lands. No reason to block this embed on that.

Merge when the Windows package job is green (your gate).