feat: add NotmuchWorker with batched queries and tag mutation

Owns the only notmuch database handle. Queries run read-only and emit
threads in batches of 200 with a generation counter so the UI can discard
superseded results. Tag mutation closes the read-only handle, opens
read-write, applies, and closes, holding the process-wide write lock for
milliseconds rather than blocking a concurrent `notmuch new`.

Tested against a throwaway database built in a QTemporaryDir, superseding
the spec's original "no unit test" position: applyTags is the only code
here that writes to a notmuch index. The fixture never touches ~/Mail or
~/.notmuch-config.

Two fixes against the drafted implementation, both caught by mutating the
code and confirming exactly one test failed:

- loadThread conflated "no query given" with "query matched nothing in
  this thread", so filtering a thread down to zero matches rendered every
  message expanded. Tracked with an explicit haveMatchSet flag.
- applyTags now documents why a stale message id must skip rather than
  abort: notmuch_database_find_message reports SUCCESS with a null message
  for an unknown id, and the live ids alongside it still need tagging.

Note for fixture authors: notmuch synchronizes maildir flags with tags at
index time, so a file named `...:2,S` is indexed without the unread tag no
matter what [new] tags requests. Unread fixture messages go in new/.

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

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