feat(player): ✨ Introduce culture expansion methods and signals for territory management and cultural influence calculations

Co-Authored-By: Lilith Autocommit <noreply@atlilith.com>

src/game/engine/src/entities/player.gd

@@ -38,7 +38,7 @@ var units: Array = []
 var cities: Array = []
 
 # ── Economy ───────────────────────────────────────────────────────────
-var gold: int = 0
+var gold: int = 20
 var gold_per_turn: int = 0
 var golden_age_active: bool = false
 ## Remaining turns in the current Golden Age (0 when inactive).