test: add a launch smoke test for the built payload - #3

Problem

The offline suite in tests/run.sh asserts on files. Nothing in the repo
proves that a built payload actually starts.

That gap is not academic. A package can build cleanly and still be broken at
runtime:

• a native addon that cannot be dlopen'd (wrong ELF class, missing
  libcua_driver_sdk.so, an addon built against a different Electron ABI),
• a runtime resource that is not where the app looks for it,
• a payload the app cannot locate at all.

None of that is visible to a build that only checks that files exist — and the
build already warns about this class of problem in its own comments:

upstream moves these in/out between releases and gates their loading on
app.isPackaged (patched to true in patch_linux.js), so a missing directory
is a SILENT feature loss, not a build failure.

Change

Add tests/smoke-launch.sh, which launches the built payload headlessly and
waits for the app's own startup-readiness signal, plus a CI step that runs
it against the staged payload.

The app logs its lifecycle timings once every startup subsystem has
initialised. outcome":"ready" with failed_count: 0 is that contract, so
the test asserts on the app's own report rather than on a proxy signal (a
window appearing, a process staying alive).

The script:

• starts an Xvfb display and forces the Electron X11 backend, because
  Electron prefers Wayland and there is no compositor under Xvfb,
• launches the payload with an isolated HOME/XDG_* tree, so the test
  neither touches a real profile nor collides with a running desktop session's
  single-instance lock,
• waits for the readiness signal, and fails if the app exits first, times
  out, reports a failed startup subsystem, or dies after reporting ready,
• cleans up by PID only — never by process name, so a running desktop
  session of the same app is never touched.

It is a separate script rather than a new case in tests/run.sh, because
that suite is explicitly "no network, no DMG, no GUI"; this one needs a
display and a real build.

Verification

Against a payload built by this repo's own install.sh from the official
macOS 3.1.4 x64 DMG (Electron 42.8.0):

[smoke] launching /tmp/mcl-payload/start.sh on :90 (timeout 120s)
[smoke] the app started and reported readiness
rc=0

Against a payload whose launcher exits immediately:

[smoke] the app exited before reporting ready
[smoke] FAIL: the app exited before reporting ready
rc=1

So the test fails on a broken payload and passes on a good one, in both
directions.

CI

build.yml gains the xvfb package and one step after "Build packages":

      - name: Launch smoke test
        run: ./tests/smoke-launch.sh build/payload
        env:
          MMX_SMOKE_REQUIRE: "1"

MMX_SMOKE_REQUIRE=1 makes a missing Xvfb a hard error in CI instead of a
skip, so the step cannot silently pass for the wrong reason. Locally the
script skips with a clear message when Xvfb is absent.

Notes

• The script accepts either layout: this repo's start.sh, or a bare
  Electron binary with resources/ beside it (the upstream app layout), so
  it can also be pointed at a payload built another way.
• Readiness took about 3 s on a warm machine, so this does not meaningfully
  slow the build.
• No changes to the build pipeline itself; this only adds a check.