Adds setupFullDeletionFixture (federated user joins remote space, authors 2
messages with reactions, opens 1-on-1 DM with a live observer). Tests #3 (soft
mode: tombstone + dm_members cleared, messages/reactions retained) and #5 (full
mode: tombstone + messages/reactions purged, surviving 1-on-1 DM channel).
Also fixes seedPeer to install both the URL form (outbound lookup on sender)
and the DOMAIN-claim form (inbound auth on receiver) — required because the
test harness's ephemeral http://127.0.0.1 origin and DOMAIN-derived
getOurOrigin() return different strings, while production has them coincide.
This was latent: test #1 (leave mode) skips S2S, so #3 was the first test to
actually exercise the S2S delete path and surfaced the dual-origin gap.
Jannis Braun
c5e3d36689