feat(build): optional xvfb-run for Linux GUI builds

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).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ci_orchestrator/commands/build.py

@@ -142,6 +142,7 @@ def _linux_build(
     artifact_source: str,
     extra_env: dict[str, str],
     skip_artifact: bool,
+    use_xvfb: bool = False,
 ) -> None:
     click.echo("[build run] Linux build mode (SSH transport)")
 
@@ -220,13 +221,21 @@ def _linux_build(
             f"export {k}={_sh_quote(v)}; " for k, v in extra_env.items()
         )
         cmd = build_command or "make"
+        # Optionally run the build under a headless X server (xvfb-run) so GUI
+        # toolkits (e.g. GTK4/PyGObject) have a $DISPLAY. The exported env vars
+        # precede xvfb-run so they propagate into the virtual-display session;
+        # -a auto-selects a free server number. The whole `cd && build` is
+        # wrapped via `sh -c` so xvfb-run drives a single command.
+        inner = f"cd {_sh_quote(workdir)} && {cmd}"
+        run_part = f"xvfb-run -a sh -c {_sh_quote(inner)}" if use_xvfb else inner
         # PYTHONUNBUFFERED so the guest python flushes promptly and the
         # streamed output is timely rather than emitted in one block.
-        full_cmd = (
-            f"{env_prefix}export PYTHONUNBUFFERED=1; "
-            f"cd {_sh_quote(workdir)} && {cmd}"
+        full_cmd = f"{env_prefix}export PYTHONUNBUFFERED=1; {run_part}"
+        xvfb_note = " [xvfb-run]" if use_xvfb else ""
+        click.echo(
+            f"[build run] running build{xvfb_note} (env vars redacted): "
+            f"cd {workdir} && {cmd}"
         )
-        click.echo(f"[build run] running build (env vars redacted): cd {workdir} && {cmd}")
         click.echo("[build run] ----- build output (live) -----")
         result = t.run_streaming(full_cmd, check=False)
         click.echo("[build run] ----- end build output -----")
@@ -508,6 +517,13 @@ def _windows_build(
 @click.option("--configuration", default="Release", show_default=True)
 @click.option("--skip-artifact", "skip_artifact", is_flag=True, default=False)
 @click.option("--use-shared-cache", "use_shared_cache", is_flag=True, default=False)
+@click.option(
+    "--xvfb",
+    "use_xvfb",
+    is_flag=True,
+    default=False,
+    help="Linux only: run the build under xvfb-run (headless X for GUI toolkits).",
+)
 @click.option(
     "--extra-env",
     "extra_env",
@@ -536,6 +552,7 @@ def build_run(
     configuration: str,
     skip_artifact: bool,
     use_shared_cache: bool,
+    use_xvfb: bool,
     extra_env: tuple[str, ...],
 ) -> None:
     """Run a build inside a guest VM, optionally packaging the result.
@@ -589,10 +606,13 @@ def build_run(
                 artifact_source=guest_artifact_source,
                 extra_env=extra,
                 skip_artifact=skip_artifact,
+                use_xvfb=use_xvfb,
             )
         except (TransportError, TransportCommandError) as exc:
             raise click.ClickException(str(exc)) from exc
     else:
+        if use_xvfb:
+            click.echo("[build run] --xvfb ignored for Windows guest.")
         workdir = guest_work_dir or "C:\\CI\\build"
         output_dir = guest_output_dir or "C:\\CI\\output"
         target = credential_target or config.guest_cred_target