item: items in unwalkable buildings are reachable from adjacent tiles

[Alistair-Afton]

Summary

Fixes DFHack/dfhack#5771

The reachable/unreachable filters only checked the walkable group of the item's own tile. Loose contents of impassable buildings (like bolts loaded in a bolt thrower) were therefore always reported unreachable even though citizens can retrieve them by standing next to the building.

When the item tile is not citizen-walkable, the check now also looks at whether the item is held by or sitting on a building that has a citizen-reachable tile in or next to its footprint. Items installed as building components (use_mode PERM: mechanisms, well buckets, built furniture) are still unreachable, as are items in wall constructions.

Testing

In a live fort:

• Moved a loose item into a well (impassable building) as TEMP content: previously unreachable, now reachable via the adjacent walkable tile.
• All unwalkable-tile held items in the fort are PERM components (statues, floodgate/well parts, trap mechanisms) and remain unreachable.
• Walkable-tile items unchanged (fast path unchanged).