test(mainwindow): give it a real worker, and probe item 66

WorkerBackedWindow builds a throwaway notmuch database, writes a config
pointing at it, and hands back a Config. Opt-in per test: the fifty-odd
existing cases construct a bare window and pay nothing.

No production change. wireWorker() already builds the worker from
m_config.notmuchConfig(), so a written qtmaildir.conf exercises the
shipping path rather than a parallel one built for tests.

Three findings, each of which had made an earlier version of this test
worthless:

The worker is unreachable by findChild. wireWorker() creates it
parentless and moves it to its own thread, so the tests wait with
QTRY_VERIFY on observable state instead of on worker signals.

rowCount on a thread row is 0 until the thread is expanded, since
children are populated by the expansion. hasChildren is the correct
pre-expansion assertion and also proves the two messages threaded.

currentThreadId() is assigned synchronously in the selection handler,
before any worker round-trip, so it reports intent rather than content:
the test passed with onThreadLoaded() disabled entirely. It asserts on
MessageView::showingPlaceholder() now, checked blank before the click so
the check after it means something, and mutation-confirmed to fail when
the load is disabled.

Item 66 does NOT reproduce here: selecting an unexpanded thread root in
a two-message thread shows the message. Recorded rather than widened.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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