feat(chat): group chats in the conversation feed

[bmc08gt]

Stack:

1. feat(chat): group chat contract and service layer #1468 — contract and service layer
2. feat(chat): group chats in the conversation feed #1469 — feed and roster (this PR)
3. feat(chat): render a group conversation #1470 — UI

Second of three, on top of feat/group-chat-contract. Groups reach the conversation feed, and their rosters stay current.

• Feed queries by page, by type and by membership, surfaced on the data sources.
• A watermark per source, so the merged DM-and-group feed pages without either side starving the other or a page boundary dropping a row.
• A group projected into the conversation list beside a DM, and the list paged off the database rather than the network.
• Roster changes applied in version order, with a refetch when the version jumps past a gap — the stream can be down, and the answer has to converge either way.
• GroupFeedDelegate beside the DM's FeedSyncDelegate, with sync, join and leave routed through the coordinator.

Membership is the load-bearing idea here: it is local state, because the two things that change it — your own join and your own removal — arrive as a roster change on a stream that can drop, and as a call the list should not wait on.

Nothing renders a group yet; that is the third PR.