fix(delete): recover when a sync renamed the file underneath a move

Item 162. mbsync uploads a file and renames it to record the server UID
(`<name>,U=<uid>:2,<flags>`), and notmuch keeps the pre-`U=` name until
that sync's `notmuch new` runs. moveMessages() renamed a path that no
longer existed, reported "Cannot move <file> to <folder>", and skipped
the message: Delete silently did nothing while blaming the destination
folder for a timing problem.

Before renaming, check whether the recorded path still exists. If it
does not, reindex that one Maildir directory and re-read the message's
filenames by id, taking the one that is on disk.

Recovery is by MESSAGE ID rather than by scanning the folder, because
two files can carry the same id and scanning could move the wrong one.
reindexFolder() indexes a single directory and is deliberately not a
`notmuch new`, which would walk the whole Maildir and run the post-new
hook that tags real mail.

Bounded to one reindex and one retry, so a file that is genuinely gone
still reports rather than becoming a silent no-op. The second test pins
that half.

Holding the move while a sync runs was the other candidate and is not
the fix: sendMove() already refuses on the write lock (items 97 and
106), but aSyncHoldsTheWriteLock() tracks notmuch's lock, while this
window sits between mbsync's rename and that sync's `notmuch new`.
mbsync renames without touching that lock, so the damaging window is
open when there is nothing to observe. That refusal is left alone; it
does its own job.

The ordinary fixture layout cannot see this, since nothing renames a
file underneath the index. The test renames without reindexing, which
is exactly the window mbsync opens, and guards that the database still
names the old path so it cannot pass against a fixture that quietly
reindexed. Mutation-checked: disabling the recovery reproduces the
original error.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UUQS6n3cmsFrsjCNmwNtf8

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