[Epic] Networking / multiplayer support

[stormmuller]

Goal

Add networking/multiplayer building blocks to the engine. There is currently no networking code anywhere in src — games needing multiplayer have to build this entirely themselves outside the engine.

Scope (suggested)

• A transport abstraction (WebSocket at minimum; WebRTC data channels for peer-to-peer scenarios is a reasonable stretch goal).
• State synchronization strategy for the ECS: likely a snapshot/delta-compression approach over EcsWorld component data, given the engine's sparse-set ECS design (src/ecs).
• Client-side prediction / server reconciliation and interpolation are the hard, engine-specific parts worth designing carefully rather than bolting on generically — consider scoping an initial release to authoritative-server state sync without prediction, and layering prediction in later.
• Clear separation so single-player games pay zero cost for this module (matches the engine's existing modular subpath-export pattern, e.g. @forge-game-engine/forge/networking).

Why this is a milestone, not a single issue

Networking touches ECS serialization, the game loop/timing model, and introduces a large new design space (authoritative vs. peer-to-peer, reconciliation strategy). This needs an initial design/RFC before implementation and will span multiple PRs.

Acceptance criteria (top-level)

• A minimal transport + state-sync path exists and is demonstrated end-to-end (e.g. a simple multiplayer demo)
• Documented in documentation-site/docs/docs