Implements the full server-side tick pipeline:
- process_player_input drains InputQueue, converts PlayerActions to
MoveIntent components or pause/unpause toggles
- generate_snapshot builds ObserverSnapshot from ECS state with
render coordinate conversion
- receive_bridge_inputs/send_bridge_snapshot handle bridge I/O with
graceful disconnect detection via ServerRunning resource
- main.rs now accepts TCP connections and runs a proper game loop
- PlayerCharacter marker, Player EntityKind, SnapshotBuffer resource
Closes server side of #81, #82, #83.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>