feat(simulation): add input processing, snapshot gen, and game loop

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>

server/src/simulation/mod.rs

@@ -20,10 +20,13 @@ impl Plugin for SimulationPlugin {
         app.init_resource::<time::SimulationTime>()
             .insert_resource(rng::SimRng::new(0))
             .init_resource::<input::InputQueue>()
-            .add_systems(Update, time::advance_tick)
             .add_systems(
                 Update,
-                movement::validate_movement.after(time::advance_tick),
+                (
+                    input::process_player_input,
+                    time::advance_tick.after(input::process_player_input),
+                    movement::validate_movement.after(time::advance_tick),
+                ),
             );
 
         tracing::debug!("SimulationPlugin initialized");