Add an --xvfb switch to 'build run' and 'job' that wraps the Linux build
command in 'xvfb-run -a sh -c ...', giving GUI toolkits (GTK4/PyGObject)
a headless $DISPLAY. Exported env vars precede xvfb-run so they
propagate into the virtual-display session. No-op (with a notice) for
Windows guests.
The local-ci-build composite action exposes it as the 'xvfb' input
(default false), forwarded as INPUT_XVFB -> --xvfb, mirroring the
use-shared-cache/skip-artifact wiring.
Docs: WORKFLOW-AUTHORING.md parameter table + Linux example note;
LINUX-TEMPLATE-SETUP.md Step 4a (the template ships python3-gi
gir1.2-gtk-4.0 xvfb). Adds build-run unit tests (wrap, plain, Windows
no-op).
Simone
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