feat: introduce identity-backed goat

tests/simulation_state_serialization_test.gd

@@ -19,8 +19,10 @@ func _run() -> void:
 	_test_previous_world_provenance_migration()
 	_test_previous_world_retention_migration()
 	_test_previous_world_opportunity_migration()
+	_test_previous_world_animal_migration()
 	_test_relationship_schema_rejection()
 	_test_opportunity_schema_rejection()
+	_test_animal_schema_rejection()
 	_test_schema_rejection()
 
 	if failures.is_empty():
@@ -481,6 +483,51 @@ func _test_previous_world_opportunity_migration() -> void:
 	manager.free()
 
 
+func _test_previous_world_animal_migration() -> void:
+	var manager := _create_manager(63)
+	var v9_data: Dictionary = _build_opportunity_world(manager, false)
+	v9_data["schema_version"] = SimulationStateRecord.ANIMAL_LEGACY_SCHEMA_VERSION
+	v9_data.erase("animals")
+	var migrated := SimulationStateRecord.from_dictionary(v9_data)
+	_check(migrated != null, "World schema v9 should add an empty animal stream")
+	if migrated != null:
+		_check(
+			migrated.animals.is_empty() and migrated.opportunities.size() == 1,
+			"World v9 migration should preserve opportunity history while adding animals"
+		)
+	manager.free()
+
+
+func _test_animal_schema_rejection() -> void:
+	var manager := _create_manager(64)
+	var missing_animals: Dictionary = manager.create_state_record().to_dictionary()
+	missing_animals.erase("animals")
+	_check(
+		SimulationStateRecord.from_dictionary(missing_animals) == null,
+		"Current world records should require the authoritative animal array"
+	)
+	var invalid_position := (
+		AnimalStateRecord
+		. from_dictionary(
+			{
+				"schema_version": AnimalStateRecord.SCHEMA_VERSION,
+				"animal_id": "invalid_goat",
+				"display_name": "Invalid",
+				"species_id": String(SimulationIds.SPECIES_GOAT),
+				"position": [NAN, 0.0, 0.0],
+				"hunger": 70.0,
+				"last_fed_tick": AnimalStateRecord.NEVER_FED_TICK,
+				"reserved_by": -1,
+				"enabled": true,
+				"can_npcs_feed": true,
+				"can_player_feed": true,
+			}
+		)
+	)
+	_check(invalid_position == null, "Animal records should reject non-finite positions")
+	manager.free()
+
+
 func _test_relationship_schema_rejection() -> void:
 	var manager := _create_manager(58)
 	var missing_cause_data: Dictionary = manager.create_state_record().to_dictionary()