E8 — Networking (server-authoritative + prediction)

[fredboy]

Goal: multiplayer-ready architecture; single-player as a co-located server.

Scope / deliverables:

• engine:net: transport + replication + snapshots + command stream.
• Server-authoritative loop at 20 TPS with subticks.
• Local-player prediction at the same 20 TPS + reconcile (60 Hz is render interpolation/smoothing only).
• Loopback transport for single-player (SP and MP share one code path).

Definition of Done: SP runs through the loopback server; prediction + reconcile converge. Snapshot-determinism + reconcile-convergence tests.

Depends on: E2, E4, E7