mautrix bridges carry full participant list in to[]

For every mautrix bridge, the normalised event 'to[]' array always carries the full participant list with each member's displayname (from cached m.room.member ghosts), minus the sender and minus the user's self-puppets on outbound. That's the strongest identity signal for group rooms — stronger than 'room_name' (which is the group title for groups, but the peer's name for bridged DMs). UIs that key on to[0] and discard the rest throw away the only data that actually disambiguates one group member from another. Also: WhatsApp group senders arrive as 'lid-<digits>' (privacy ID), not phone — needs a LID→phone resolver backed by mautrix-whatsapp's whatsmeow_lid_map.