bundled: SIGTERM the frankenphp child via aboutToQuit, not just the dtor

Symptom (user report on v0.1.1):

  QProcess: Destroyed while process ("/tmp/.mount_Todo-xbNoHHL/usr/bin/frankenphp") is still running.

…and the frankenphp child + its PHP workers were left orphaned after
the host exited.

Cause: teardownChild() was only called from ~BackendConnection. By
the time that destructor runs, app.exec() has already returned,
QQmlApplicationEngine is mid-destruction, and Qt's event loop is
half-torn-down. waitForFinished() doesn't reliably reap the child in
that window — QProcess gets destroyed by the QObject parent-chain
cleanup before the kernel reports the child as exited.

Fix: in BackendConnection's constructor, connect
QCoreApplication::aboutToQuit → teardownChild. aboutToQuit fires
while the event loop is still active and BEFORE main() starts
unwinding the stack, so SIGTERM + waitForFinished can do their job
properly. The destructor's teardownChild call stays as belt-and-
suspenders (no-op once aboutToQuit has already cleaned up — the
function is idempotent via the m_child = nullptr at its end).

The connect happens unconditionally in the constructor (not just for
bundled mode) because m_child is also nullptr in dev mode and
teardownChild handles that with its leading `if (!m_child) return;`.

Regression guard: examples/todo/tests/bundled-supervisor.sh gains a
"graceful shutdown" step:

  - Snapshots the host's child PIDs before SIGTERM
  - SIGTERMs the host, waits up to 3s for clean exit
  - Greps the host log for "QProcess: Destroyed while" — fail if found
  - Iterates the snapshotted PIDs, fails on any frankenphp orphan still alive

Verified locally: real AppImage + the integration test both clean up
without Qt warnings or orphan processes.

PLAN.md: new v0.1.2 section above v0.1.1, this is its first entry.
CHANGELOG.md: [0.1.2] — TBD section with the same Fixed entry.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

PLAN.md

@@ -531,6 +531,12 @@ Per-phase scope detail is preserved in `CHANGELOG.md` (per-version summary) and
 
 First public preview. Linux AppImage. Full entry in `CHANGELOG.md`; binaries at <https://src.bundespruefstelle.ch/magdev/php-qml/releases/tag/v0.1.0>.
 
+### v0.1.2 — next bugfix
+
+In-flight on `dev`:
+
+- **Bundled supervisor: clean child shutdown.** The destructor's `teardownChild()` only ran during stack unwinding *after* `app.exec()` returned, by which point Qt's event loop was already mid-shutdown — so `QProcess::waitForFinished` couldn't reliably reap the child and Qt warned `QProcess: Destroyed while process is still running`, leaving an orphan frankenphp + its workers behind. Fix: connect `QCoreApplication::aboutToQuit` to `teardownChild` in the constructor, so the child is SIGTERM'd while the event loop is still active. Bundled-supervisor integration test gained a clean-shutdown assertion (no Qt warning, no orphan frankenphp under the host's PGID after SIGTERM).
+
 ### v0.1.1 — bugfix release (ready to tag)
 
 All four shakedown follow-ups landed: