Skip known-unregistered recipients when sending to groups (#2077)

Group sends built the recipient list from the full group membership, so
members already known to be unregistered were retried on every send via
the legacy 1:1 fan-out. On large groups this made a single send take
tens of seconds and could time out, leaving the message undelivered.

Filter out recipients whose unregistered timestamp is set before
sending, returning an unregisteredFailure result for each (so callers
and CLI output are unchanged) without the network attempt. The flag is
maintained independently by profile/CDS discovery and is cleared when a
recipient registers again, so skipped recipients are re-included
automatically.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>