Refactor Owniverse modules for quantity mathematics and entity catalog

- Introduced `quantity_math.gd` for arithmetic operations including allocation, normalization, and accumulation of quantities without scene tree dependencies.
- Created `entity_catalog.gd` to manage entity definitions, groups, and event indexes, ensuring separation from simulation state and UI.
- Implemented `item_presentation.gd` for read-only projections of simulation state, handling amounts and unlock conditions for UI elements.
- Updated `civilization_evolution.gd` and `civilization_state.gd` to utilize new quantity math functions, improving code clarity and maintainability.
- Enhanced tests in `test_quantity_catalog.gd` to validate new functionality and ensure correctness of mathematical operations.
- Revised documentation to reflect changes in module structure and functionality.

world/owniverse/civilization_state.gd

@@ -1,5 +1,7 @@
 extends RefCounted
 
+const QuantityMath = preload("res://world/owniverse/quantity_math.gd")
+
 const ETHICS = ["m", "l", "w", "d", "c", "b"]
 
 static func is_space_civilization(id: String, rules: Dictionary) -> bool:
@@ -19,7 +21,7 @@ static func reconcile(world, target: Dictionary, rules: Dictionary) -> void:
             current[ethic] = maxf(0.0, current.get(ethic, 0.0))
             total += current[ethic]
         if total > count:
-            current = world._allocate_whole(current, count)
+            current = QuantityMath.allocate_whole(current, count)
         elif total < count:
             var missing: float = count - total
             var base: float = floorf(missing / ETHICS.size())
@@ -131,7 +133,7 @@ static func enforce_capacity(world, target: Dictionary, rules: Dictionary, year:
         target.amount[id] = supported
         world._credit(target, previous, excess)
         world._append_batch(target, previous, excess, year, year)
-        var removed: Dictionary = world._allocate_whole(target.ethics[id], excess)
+        var removed: Dictionary = QuantityMath.allocate_whole(target.ethics[id], excess)
         if not target.ethics.has(previous):
             target.ethics[previous] = {}
         for ethic in ETHICS:
@@ -143,7 +145,7 @@ static func enforce_capacity(world, target: Dictionary, rules: Dictionary, year:
             for star in physical:
                 if world.entities[star].event_group == "DwStar" and physical[star] > 0.0:
                     stars[star] = physical[star]
-            var weights: Dictionary = world._normalize_distribution(stars)
+            var weights: Dictionary = QuantityMath.normalize_distribution(stars)
             # Worlds are aggregates: a star may host several inhabited planets.
             var assigned: float = 0.0
             var owners: Array = weights.keys()