ci: prove the Xvfb display before desktop jobs and smoke artifacts once

[Rinat S (rsolmano)]

Problem

On the first run of #642 (now merged), desktop shard 1/4 failed before any test ran. The native smoke:desktop timed out (native-ui desktop ready, 30 s) because the virtual display never became usable: Electrobun logged GlobalShortcut event loop failed to initialize and GTK never started, where a healthy run logs X11 display opened successfully. That is a runner-environment fault, not an app regression. With the suites sharded, each native smoke also runs in every shard, so a rare fault like this is 4× as likely to fail CI while adding no coverage.

Approach

Retry the virtual display only, never the app or the tests.

• Desktop shards start Xvfb as an explicit step and continue only once xdpyinfo (an X client on the exported DISPLAY, no auth cookie) connects. If it doesn't answer within 10 s, the server is killed, its lock and socket are cleared, and it is restarted, at most 3 times. After that the job fails with the Xvfb log. Once the display is proven, any application failure fails CI as before.
• smoke:binary and smoke:desktop run only in shard 1. Each run still smokes each artifact exactly once, as it did before sharding.

Blanket job retries were considered and rejected: they would also hide real regressions, against the no-blanket-retries flake policy in e2e/SPEC.md.

Changes

• .github/workflows/ci.yml
  • New Start virtual display step in desktop-shard that exports DISPLAY=:99 via GITHUB_ENV.
  • xvfb-run -a dropped from the smoke and e2e steps, and x11-utils replaces xauth in the apt list.
  • if: matrix.shard == 1 on smoke:binary and smoke:desktop.
• .github/SPEC.md: records the shard-1-only smoke rule and the display-only retry boundary.

Screenshots

Not applicable — no frontend changes.

Related issues

None.

Checklist

• Fast gates pass: bun run lint, bun run typecheck, bun run test — not rerun: no source, test or config files changed (CI workflow + spec only); this PR's CI runs them
• E2E suite passes for app-affecting changes (bun run e2e, or bun run e2e:full when touching agent behavior) — not app-affecting; this PR's CI runs every desktop/binary shard on the new display step
• Before/after screenshots are included for frontend changes, or marked not applicable
• Relevant SPEC.md / top-level specs updated to reflect any boundary, contract, or behavior change
• I have read the Contributing guide and agree to the Code of Conduct

Testing

• @action-validator/cli .github/workflows/ci.yml → valid
• bash -n on the extracted display step → OK
• Display step with stubbed Xvfb/xdpyinfo, server answering on the 2nd start → exit 0 after 2 starts, DISPLAY=:99 exported
• Same stubs, server never answering → exit 1 after 3 starts, nothing exported
• Not run on a Linux runner locally; this PR's CI is the first real run of the display step.

[jetbrains-air]

Approved — ready to merge.

[jetbrains-air]

Approved — ready to merge.

[jetbrains-air]

Approved — ready to merge.