feat(sonar): add SAND terrain material

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

game/domain/sonar/sonar_material.gd

@@ -5,8 +5,13 @@ enum Value {
 	UNKNOWN,
 	ROCK,
 	METAL,
+	SAND,
 }
 
 
 static func is_valid(value: int) -> bool:
-	return value >= Value.UNKNOWN and value <= Value.METAL
+	return value >= Value.UNKNOWN and value <= Value.SAND
+
+
+static func is_terrain(value: int) -> bool:
+	return value == Value.ROCK or value == Value.SAND