feat(engine): ✨ Add Building and Improvement entity classes with core mechanics and properties

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

engine/src/entities/building.gd

@@ -1,2 +1,46 @@
 class_name Building
 extends RefCounted
+## Game entity for both city buildings and map-placed NPC buildings (lairs, villages, ruins).
+## City buildings: placement="city", owner=player_index, position=city tile.
+## NPC buildings: placement="map", owner=-1, position=map tile.
+
+## Unique instance ID (e.g. "lair_42", "building_granary_3")
+var id: String = ""
+## Building type ID from data (e.g. "beast_den", "granary", "village", "ruin")
+var type_id: String = ""
+## Display name resolved from data
+var name: String = ""
+## "map" for NPC buildings placed on the world map, "city" for city buildings
+var placement: String = "city"
+## Owning player index. -1 = unowned (NPC / world)
+var owner: int = -1
+## Map position (axial coordinates). Meaningful for placement="map".
+var position: Vector2i = Vector2i.ZERO
+## Whether this building has been visited/explored by a player
+var visited: bool = false
+
+
+static func from_dict(data: Dictionary) -> Building:
+	var b := Building.new()
+	b.id = data.get("id", "")
+	b.type_id = data.get("type_id", "")
+	b.name = data.get("name", "")
+	b.placement = data.get("placement", "city")
+	b.owner = data.get("owner", -1)
+	var pos: Array = data.get("position", [0, 0])
+	if pos.size() >= 2:
+		b.position = Vector2i(int(pos[0]), int(pos[1]))
+	b.visited = data.get("visited", false)
+	return b
+
+
+func to_dict() -> Dictionary:
+	return {
+		"id": id,
+		"type_id": type_id,
+		"name": name,
+		"placement": placement,
+		"owner": owner,
+		"position": [position.x, position.y],
+		"visited": visited,
+	}

engine/src/entities/improvement.gd

@@ -105,7 +105,7 @@ static func _get_data(improvement_type: String) -> Dictionary:
 	return FALLBACK_DATA.get(improvement_type, {})
 
 
-static func can_build(improvement_type: String, terrain_id: String) -> bool:
+static func can_build(improvement_type: String, biome_id: String) -> bool:
 	## Check if this improvement can be placed on the given terrain.
 	var data: Dictionary = _get_data(improvement_type)
 	if data.is_empty():
@@ -115,10 +115,10 @@ static func can_build(improvement_type: String, terrain_id: String) -> bool:
 
 	# "any_land" allows any non-water terrain
 	if valid is String and valid == "any_land":
-		return terrain_id != "ocean"
+		return biome_id != "ocean"
 
 	if valid is Array:
-		return terrain_id in valid
+		return biome_id in valid
 
 	return false