[Feature] Groups x devices (Stage 3e): deliver group messages to members' linked devices

[ThePhantomPuppet]

Group fan-out and self-sync are currently 1:1-only:

• A group message reaches only a member's primary device.
• My own second device never sees group messages I send or receive.

Move the group send path from sendContent (single pinned device) to
fanoutDeliveries (per-device). Also handle attachment cardinality
(Sigma devices x members) as an availability lever.

Executable target already in the repo: tests/group-device-fanout.xfail.test.mjs.