fix(tabs): move the reused draft to a reopened draft's slot

Reopening a closed draft resolves to the target group's existing draft
(the per-group singleton) rather than a tab of its own, so the recorded
slot went unused: walking back a close-all over `[conv, draft, conv]`
rebuilt it as `[conv, conv, draft]`, with the replacement draft shunted
to the end of the strip it was meant to restore. The reuse path now
moves that draft to the slot, through all three of its branches.

Also covers the file strip's batch closers end to end: close-others and
close-all there had only the `batchCloseSlots` unit test behind them.