fix(compose): resolve a path a sync renamed, at all three read sites

Item 163. mbsync renames an uploaded file to add its `,U=<uid>` infix,
and the model's `MessageRef::filePath` was captured when the query ran,
so a row loaded before that sync names a file that no longer exists.
MimeParser then honestly reports a message it cannot open.

MaildirName::resolveRenamed() answers the filesystem question: returns
the path unchanged when it still exists, otherwise looks in that one
directory for the file whose unique stem matches. mbsync preserves the
stem (`<stem>:2,D` becomes `<stem>,U=5:2,D`), which is what makes this
safe to do by filename at all. It never recurses, never crosses a folder
boundary, and refuses an ambiguous match rather than guessing, since
opening or moving the wrong message is worse than reporting none.

It lives in MaildirName because that namespace already owns the `,U=`
infix and is a pure-value unit testable without a widget. A file that
changed FOLDERS is a different question that only the message id can
answer, and NotmuchWorker::moveMessages() re-resolves that way already.

Three call sites, all of which held a stale path:

- The message pane, which reported "(unreadable message)" over a file
  that was on disk and readable. Cosmetic and self-repairing.
- Reply and Forward, refused outright, so the user could not answer a
  message that was sitting there.
- The draft reopen, and this is the half that costs data. The refusal
  happens BEFORE any composer exists, so the user composes again into a
  fresh window whose autosave has no previous path to unlink. The old
  revision survives, each save mints a new Message-ID, and both files
  reach the server. The unlink machinery was correct throughout and
  never ran.

forDraft() seeds draftPath from the RESOLVED path, never the caller's:
seeding the stale one would let the reopen succeed and the unlink still
miss, which is the same fork arriving one step later.

Covered by five unit tests on the resolver, including the two that keep
it honest (a genuinely missing file yields nothing, and a neighbouring
message is never matched), and by an integration test that renames the
draft the way mbsync does and asserts the file COUNT, which is the shape
the fork actually takes. Both mutation-checked; the integration test
fails with the reported symptom when the resolution is removed.

The stable-Message-ID question is deliberately untouched: it is what
turns a stale path into two server-side messages rather than one
replaced file, and it wants its own item.

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

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