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.

autoload/global.gd

@@ -59,8 +59,23 @@ func get_description(item_id: String, items: Array = []) -> String:
 
 func format_number(value: float, digits: int = 0) -> String:
     if value == 0: return ""
-    if value < 10_000:
-        return "%.*f" % [digits, value]
+    # Keep ordinary quantities readable, grouping thousands with spaces.
+    # Scientific notation is reserved for values whose fixed form would be
+    # unwieldy in the UI.
+    if absf(value) < 1_000_000_000.0:
+        var fixed: String = "%.*f" % [digits, value]
+        var parts: PackedStringArray = fixed.split(".", true)
+        var integer: String = parts[0]
+        var sign: String = ""
+        if integer.begins_with("-") or integer.begins_with("+"):
+            sign = integer.left(1)
+            integer = integer.substr(1)
+        var grouped: String = ""
+        while integer.length() > 3:
+            grouped = " " + integer.right(3) + grouped
+            integer = integer.left(integer.length() - 3)
+        grouped = sign + integer + grouped
+        return grouped + ("." + parts[1] if parts.size() > 1 else "")
 
     var exp_value = int(floor(log(abs(value)) / log(10.0)))
     var mantissa = value / pow(10.0, exp_value)