fix(delete): repair seven defects in the move-to-trash path

Item 103's implementation was committed unreviewed and never hand-tested.
Reviewing it, and then hand-testing it against real mail, found seven
defects. Six of them lose or corrupt state and none was caught by the
suite, which was green throughout.

**Undo pushed a command instead of consuming one.** onMessagesMoved()
pushed a MoveCommand for every confirmed move, including the move an undo
had just made, so undoText went "Delete", "Undo Delete", "Undo Undo
Delete". A second press of undo re-deleted the message the first had
rescued. PendingMove carries a fromUndo flag, which has to survive the
queued round trip and so cannot be a window-wide "am I undoing" flag.

**Held moves were invisible to the quit guard.** pendingEditCount() summed
the held tag edits and not the held moves, so a Delete pressed during a
sync left the count at zero: the indicator stayed hidden and closeEvent()'s
guard never fired, discarding the move on quit with no prompt. That is item
106's data loss with a worse shape, because a dropped move leaves the file
in the folder the user asked it out of.

**Two moves to one folder dropped the second's tags.** m_pendingMoves was
keyed on the destination, so two Deletes in one account before the first
confirmation both named `acct/Trash` and the second insert overwrote the
first. That file reached the trash carrying neither `deleted` nor
`deleted-from:`, unrestorable and invisible to a `tag:deleted` query. It is
a FIFO now: the worker moves one batch at a time and emits in request
order, so position alone matches a confirmation to its request.

**Second Delete left the origin tag behind.** The restore passed the origin
PLACEHOLDER in its removal list, and onMessagesMoved() resolves that from
the folder the worker reports, which on a restore is the trash. It asked to
remove `deleted-from:Trash`, a tag never written, while the real
`deleted-from:inbox` was never named. A restore does not need the
placeholder: it already read the origin to decide where to send the file.
originTagFor() is now the one derivation both sides use.

**Ctrl+Z left it behind too**, for a different reason: MoveCommand was
constructed with the unresolved pending.add. The command carries the
resolved tags now, and is pushed per origin group rather than once per
batch, because the placeholder resolves to a different tag per origin.

**A thread root re-deleted itself.** everySelectedRowHasTag() asked a
thread row about its THREAD's tags, which notmuch gives as a union. Delete
the root of a three-message thread and the replies are untouched, so the
union carries no `deleted` and a second press ran Delete again: the message
moved trash-to-trash and came out with `deleted`, `deleted-from:inbox` AND
`deleted-from:Trash`, with no way back. The union was a documented
approximation, called bounded because the worst case for a TAG toggle was
re-applying a tag the message already had. A MOVE re-applies the move.

Resolved through messageById(), NOT through ThreadSummary::firstMessageTags,
which is the value the query delivered and is never refreshed by an
optimistic update: after a delete the node reads `deleted` while the
summary still reads `unread`.

**Delete thread never moved anything.** It was left calling tagSelected()
when Delete became a move, so a whole conversation sat in the inbox wearing
a `deleted` chip. It moves every message now, each with its own origin, so
a thread spanning folders reassembles on restore. A reply row resolves to
its own thread through selectedThreadIds(): scopeFor() reports a reply
under messageIds and leaves threadIds empty, which made a thread action on
a reply row do nothing at all.

**And the root card did not repaint** until it was clicked, while its
replies did. sendMove() had no optimistic update at all, so nothing moved
until the worker answered; and applyMessageTagChange() deliberately leaves
a multi-message thread's SUMMARY alone, which is correct for a one-message
edit and wrong for a thread-scoped one. The replies have nodes and
repainted; the root card reads the summary. The thread paths repaint
synchronously with applyTagChange() before the worker is asked, which also
keeps the toggle's direction readable for the next press.

Every fix carries a test and every test was mutation-checked. Three false
greens were found while writing them and are recorded at their assertions:
a disjunction that emptied on the wrong term, a QTRY_VERIFY(rowCount() == 0)
satisfied by the interval before the worker answers, and a query issued
before the confirming write had landed. Absence is asked of notmuch
directly through a new notmuchCount() helper for that reason.

Two bare-window tests moved off assertions about synchronous pending writes
onto the model, since the thread actions now round-trip through the worker.

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

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