test: assert a move that relocates nothing writes no tag

The spec's ordering bullet had only half a test. moveMessagesReportsOnlyWhatMoved()
covers the worker refusing to report a message it did not move; nothing covered
the window writing tags only for what the worker reported.

The failure is provoked by putting a FILE where the trash folder must go, so
the Maildir subdirectories cannot be created under it. A read-only directory
would be ignored by a test running as root, which this must not depend on.

A tag written anyway is the exact half-done state item 103 removes: a message
marked deleted, its file still in the inbox, and its origin tag lying about
where it went. The mutation putting that back fails this test.

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

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