Fix O(N^2) performance degradation in form_from_map

[IcarusOfTroy]

Summary

Performance "Fix O(N^2) performance degradation in form_from_map"

Purpose of change

Fixes #88525.
The game would hang or severely degrade in performance when there were a lot of items around, particularly when opening menus that list items such as crafting or vehicle inspection.

Describe the solution

Hoists the bulk item batching out of the per-tile loop in inventory::form_from_map. Previously, add_items_bulk was called once for every tile containing more than one item, causing severe slowdowns when surrounded by many items, as it rebuilds an index of the entire accumulated inventory each time. Now, it gathers all items across all tiles into a single batch and calls add_items_bulk exactly once at the end of the function.

Describe alternatives you've considered

N/A

Testing

Compiled and checked that form_from_map correctly gathers all items into the single bulk batch, restoring expected performance and avoiding the O(N^2) stack sweeps.

Added a new unit test tests/inventory_test.cpp containing inventory_form_from_map_bulk_batching. This programmatic test proves that form_from_map properly pools items from multiple coordinates into a single bulk batch with correct totals, guarding against future regressions in the inventory loading pipeline.

Additional context

None