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/movement.rs

@@ -10,6 +10,10 @@ use std::collections::HashMap;
 /// Chunk size in tiles (32x32 per chunk)
 pub const CHUNK_SIZE: i32 = 32;
 
+/// Marker component identifying the player-controlled entity.
+#[derive(Component, Debug)]
+pub struct PlayerCharacter;
+
 /// Tile position component for grid-based movement.
 /// Discrete integer coordinates used in simulation; converted to f32
 /// at the bridge boundary for VisibleEntity wire format.