[Feature] Multiplayer: sessions, replication and transports

[miogds]

Goal

Two or more players in the same game: side by side in one room on their headsets, or apart on a FaceTime call with spatial Personas, or over the network from a Mac or an iPhone, seeing the same scene, the same zombies and each other's actions at the same moment. The engine has no networking today. CoolSaber proves the first form on its own: a SharePlay duel between two Vision Pros through GroupActivities, with a shared group-immersive-space origin and the blade state synced by code that lives inside the demo, so every other game would have to write the same thing again.

The goal is a multiplayer layer in the engine that a game turns on rather than builds: a session (who is in, joining and leaving, the shared origin in a room), replication of entity state marked as networked (transforms, animation goals, physics ownership, plugin values) with snapshots, deltas and interpolation so remote entities move smoothly, an authority model (one host or a server owning the simulation, clients predicting their own player), events between peers for gameplay, and a transport chosen per situation: SharePlay for visionOS sessions with Personas and voice for free, the local network for a room without FaceTime, a dedicated server later for remote play at scale. The editor marks what is replicated; the same scene runs single-player unchanged.

Status

Step	Plan	Status
Proof in a demo	SharePlay duel with spatial Personas, shared origin, blade state sync	Done in CoolSaber (its own code)
Requirements	Co-located and remote forms, player count, what the game replicates (players, zombies, physics props), authority model, latency budget on visionOS	Not started
Session	Join and leave, participant identity, shared spatial origin, reconnection	Not started
Transport	GroupActivities first (visionOS, iOS, macOS), local network second, dedicated server later, behind one interface	Not started
Replication	Networked component marking, snapshots and deltas, interpolation and extrapolation, ownership transfer	Not started
Authority and prediction	Host or server simulation, client-side prediction for the local player, reconciliation	Not started
Physics and animation	Jolt state ownership per body, motion-matching goals replicated instead of poses	Not started
Editor	Mark entities as replicated, session debug panel	Not started
Sample	CoolSaber's sync moved onto the engine layer; a two-player zombie room	Not started

Next steps

• Requirements note, then session and replication over GroupActivities, with CoolSaber as the first migration.
• Depends on the physics seam for body ownership and on the motion-matching extensions for goal replication.