refactor: drop the unnettable pending-edit counter

Item 119's stated blocker, removed by finding out what it held: nothing.

pendingEditCount() summed four sources, three of which can name the messages
they hold and one of which was a bare int. That int counted confirmed
changes carrying no message ids, on the reasoning that an edit which cannot
be netted must still register rather than be lost. It was what made the
count impossible to open and list, since a dialog would have shown three
groups and then owed the user a remainder it could not describe.

The remainder is empty. NotmuchWorker::applyTags() is the only emitter of
tagsApplied(), and its first statement returns on an empty id list, which is
the exact condition the counter required. applyTagsToThreads() resolves
threads to message ids through a query and errors out when that comes back
empty, so it can only ever hand applyTags() a non-empty list.

Measured rather than read. A qFatal in the branch fired in 4 of 70
test_mainwindow cases, all four building a TagChange by hand and invoking
the slot directly with no worker involved; an assertion before the worker's
own emit never fired across the whole suite, worker-backed tests included.

The worker's guard stays and is pinned where it lives, by
applyTagsWithNoIdsDoesNothing() in test_notmuchworker. The MainWindow test
that asserted the deleted branch is replaced by one for the consequence: a
change reaching the indicator names its messages, and an edit with its
inverse nets back to nothing, which is the property a growing-only counter
could never have. Three tests that leaned on the counter to show the
indicator now carry message ids, as a real edit always does.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P88Q3MCSCSQxKDy7pmXh9F

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