feat(simulation): add 8-directional movement

Add diagonal PlayerAction variants (MoveNortheast, MoveNorthwest,
MoveSoutheast, MoveSouthwest) and TilePosition::all_neighbors()
returning all 8 surrounding tiles. Genre-expected for immersive sim.

Establishes the movement pattern before pathfinding is built on top.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/tests/serialization.rs

@@ -58,6 +58,10 @@ fn all_player_action_variants_roundtrip() {
         PlayerAction::MoveSouth,
         PlayerAction::MoveEast,
         PlayerAction::MoveWest,
+        PlayerAction::MoveNortheast,
+        PlayerAction::MoveNorthwest,
+        PlayerAction::MoveSoutheast,
+        PlayerAction::MoveSouthwest,
         PlayerAction::Interact,
         PlayerAction::UsePerceptionMode("thermal".to_string()),
         PlayerAction::Pause,