ci(Linux GUI): add GLFW/OpenGL build and Xvfb smoke coverage

[cervantesh]

Help wanted

Linux CI currently tests and packages only the headless command. We need a GUI lane that installs the real X11/OpenGL prerequisites, compiles the GLFW frontend, and performs a software-rendered smoke under Xvfb.

Suggested Ubuntu dependencies

Confirm and minimize this list rather than copying it blindly:

sudo apt-get update
sudo apt-get install -y build-essential pkg-config xorg-dev libgl1-mesa-dev mesa-utils xvfb

Required commands

go version
go env GOOS GOARCH CGO_ENABLED
go run ./scripts/check-maturity-gates.go
go test ./... -count=1
go test -tags glfw ./internal/fontglyph ./internal/frontend/glfwgl ./cmd/cervterm -count=1
go vet -unsafeptr=false ./...
go build -tags glfw -o dist/cervterm-linux-gui ./cmd/cervterm
xvfb-run -a ./dist/cervterm-linux-gui --doctor
xvfb-run -a ./dist/cervterm-linux-gui --print-default-config > dist/cervterm.lua
LIBGL_ALWAYS_SOFTWARE=1 xvfb-run -a ./dist/cervterm-linux-gui --doctor

Retain the existing headless zip and PTY coverage. Add the VT parser fuzz smoke if it is not already run in the Linux lane.

Acceptance criteria

• Pull requests compile and test the Linux GLFW/OpenGL frontend.
• CI records distro, kernel, architecture, CGO, Mesa/OpenGL renderer, and relevant dependency versions.
• Xvfb/llvmpipe can initialize diagnostics without crashing.
• Headless and GUI builds are separate and clearly named.
• Build/test artifacts and logs are uploaded.
• No privileged display server or real GPU is required for the mandatory lane.
• Platform-specific skips are narrowly scoped and explained.
• A follow-up matrix strategy for Fedora/Arch dependency drift is documented.

Evidence requested

PR link, successful Actions run, dependency list, glxinfo -B or equivalent output, job duration, and any quarantined/flaky test explanation.

[cervantesh]

Tracked as the Linux GUI CI/Xvfb baseline in #118.