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/src/bridge/types.rs

@@ -47,13 +47,17 @@ pub struct PlayerInput {
     pub action: PlayerAction,
 }
 
-/// Player action variants
+/// Player action variants — 8-directional movement for immersive sim genre expectations
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub enum PlayerAction {
     MoveNorth,
     MoveSouth,
     MoveEast,
     MoveWest,
+    MoveNortheast,
+    MoveNorthwest,
+    MoveSoutheast,
+    MoveSouthwest,
     Interact,
     UsePerceptionMode(String),
     Pause,