Stacks Not Slots

Changed

• Capacity accounting now supports every positive item max-stack size with exact proportional costs. A complete legal stack always costs 64 units, so individual 128-stackable items cost 0.5 units and 96-stackable items cost exactly 2/3 of a unit.
• Capacity acceptance, category pickup limits, slot replacement, invariant repair, commands, percentages, and the browser capacity display now use exact rational arithmetic without floating-point drift.
• Added exact capacity values to the public API while retaining conservative rounded whole-unit methods for source compatibility.