Cross Dimension Distance Fix + Inventory Has Function

[KascadeKyle]

Summary

Fixes cross-dimension distance handling and corrects InventoryState::has() validation.

Changes

• Distance functions now return an unreachable sentinel when dimensions differ to prevent unintended interactions.
• InventoryState::has() now validates using both item_id and item_type instead of only item_id. This prevents items and cargos that share the same id from being counted interchangeably.
• Minor idiomatic cleanup (is_empty() replaces len() == 0, no behavioral change - but avoids linter complaint).