docs: add manual desktop smoke test instructions to README and update CI test script

- Introduce a new section in README detailing the manual desktop smoke test process for the application.
- Include commands for setting up and running the smoke test, along with expected outcomes and logging details.
- Update the CI test script to include the new smoke test script in the shellcheck validation process.

README.md

@@ -268,6 +268,18 @@ distrobox enter --no-workdir captioneer-dev -- bash -lc '
 
 Run this from the repository root. The build and run helpers resolve that path automatically.
 
+### Manual desktop smoke test
+
+`scripts/smoke/desktop-smoke.sh` boots a nested headless Wayland desktop (sway) inside the `captioneer-dev` Distrobox, routes synthesized `espeak-ng` speech through a temporary PipeWire null sink, runs the real `build/captioneer-desktop` binary against it, and fails on ERROR diagnostics, an unclean shutdown, or an empty caption transcript.
+
+```bash
+./scripts/download-models.sh      # once, if models/ is missing
+./scripts/distrobox/build.sh      # produce build/captioneer-desktop
+./scripts/smoke/desktop-smoke.sh
+```
+
+Exit `0` means pass; a report is written under `build/smoke/<timestamp>/report.txt` together with the application logs and caption transcripts. The harness installs `sway` and `espeak-ng` into the Distrobox on demand and restores the previous default sink when it finishes. If VAD recognition of synthetic speech proves flaky, rerun with `CAPTIONEER_SMOKE_MODE=fixed`. This test is manual-only and intentionally not part of CI: it needs downloaded models, a nested compositor, and live audio routing.
+
 ## Gitea Actions and releases
 
 The `Tests` workflow runs the normal and GTK-tagged Go tests, `go vet`, and ShellCheck for every pull request and branch push. To make it mandatory for pull requests, open the repository's **Settings → Branches**, edit the protection rule for `master`, enable required status checks, and select **Tests / Go and GTK tests** after its first run. Workflow files can report the check, but the branch-protection rule is what prevents merging when it fails.