docs: record manual verification items 16-20

Tag mutation (16-18) verified against three low-stakes 2023 notifications,
with the index inspected before and after every step and the mailbox
returned to its exact starting state. Bulk archive resolved two threads in
one combined query, and undo re-resolved thread ids while those threads were
outside the displayed result set, which is the case that design exists for.

Sync (19-20) verified with the real mailsync.sh. start() does not block, a
concurrent run is refused, and a run colliding with the flock returns exit 1
in 2 ms without corrupting anything.

One caveat recorded rather than fixed: the script redirects its output to
its own logfile, so MailSync captures nothing and the log pane stays empty
even on failure. qtmaildir shows what the command emits and this command
emits nothing; piping instead of redirecting in mailsync.sh would populate
the pane with no change here.

Twenty-one of twenty-two items now verified.

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

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