Unified server-derived participant presence — client surface (query + events) #13

Client-side surface for participant presence. Multi-party sessions need "who is here right now" — including quiet participants. agent.presence is agent-plane liveness, not participant presence; and an events-only feed can't show a late joiner the participants who are already connected but silent. Presence needs query + events.

Proposal (client half)

• k.presence(filter?) → current roster snapshot (thin wrapper over a governed .presence read; filter.type = user/bot/agent/…).
• A reactive presence binding for join/leave/drop deltas, reconciled by Ck-Seq against the snapshot cursor — reusing the existing snapshot + bind pattern.
• No client-side roster synthesis; agent.presence stays distinct.

Depends on

pgCK exposing a server-derived, type-filterable .presence read + join/leave/drop events (a new substrate capability — presence is live-connection truth, not a sealed instance). Gated on the styk-tv/pgCK presence issue.

Done-when

• A joining client gets the full roster incl. quiet users from one .presence call.
• Live via events with no gaps (Ck-Seq reconciliation); type filter works from the single affordance.

Relates

• Same server-derived principle as Derive dispatch identity server-side — retire the client-built id-segment #12.