Refactor save manager validation and update state structure

- Updated the save manager to enforce a new format version (2) and improved validation checks for the save data structure.
- Enhanced the `validate_data` function to ensure all required fields are present and valid.
- Adjusted the handling of legacy saves and introduced new checks for various state fields.
- Improved number formatting in the global script for better readability of large numbers.
- Updated tests to reflect changes in the save manager and validation logic, ensuring compatibility with the new state structure.
- Added new tests for group indexing and production transactions to ensure correctness of entity management and resource production.

tests/test_group_index.gd

@@ -0,0 +1,58 @@
+extends SceneTree
+const Clock = preload("res://world/owniverse/simulation_time.gd")
+var checks: int = 0
+var failures: int = 0
+
+func _initialize() -> void:
+    call_deferred("run")
+
+func check(condition: bool, message: String) -> void:
+    checks += 1
+    if not condition:
+        failures += 1
+        push_error(message)
+
+func fixture():
+    var world = load("res://world/owniverse/owniverse.gd").new()
+    var global: Node = root.get_node("Global")
+    for id in global.OwniverseEntities:
+        var entity = load("res://world/owniverse/owniverse_entity.gd").new(global.OwniverseEntities[id])
+        world.entities[id] = entity
+        world.state.amount[id] = 0.0
+        world.state.stats.total_amount[id] = 0.0
+        if entity.group_as != "":
+            world.state.amount[entity.group_as] = 0.0
+            world.state.stats.total_amount[entity.group_as] = 0.0
+    world.entities["N"].life_product = {}
+    world.entities["N"].product_per_year_type = "H"
+    world.entities["N"].product_per_year_amount = 10.0
+    world.entities["N"].auto = 0.0
+    world.state.amount.N = 1.0
+    return world
+
+func dispose(world) -> void:
+    for entity in world.entities.values():
+        entity.free()
+    world.free()
+
+func run() -> void:
+    var world = fixture()
+    for id in world.entities:
+        var expected: Array = []
+        for key in world.entities:
+            if world.entities[key].group_as == id:
+                expected.append(key)
+        check(world._is_group(id) == (not expected.is_empty()), "Group classification: " + id)
+        check(world._members(id) == (expected if not expected.is_empty() else [id]), "Group members: " + id)
+    var members: Array = world._members("Star")
+    members.clear()
+    check(not world._members("Star").is_empty(), "Returned members are independent")
+    world.entities.N.group_as = "TestGroup"
+    world.rebuild_group_index()
+    check(world._members("TestGroup") == ["N"], "Explicit definition reload")
+    world.entities.N.free()
+    world.entities.erase("N")
+    check(not world._is_group("TestGroup"), "Removed definition invalidates index")
+    print("Group index: %d checks, %d failures" % [checks, failures])
+    dispose(world)
+    quit(1 if failures else 0)