fix: Fill with copies assigns sequential names in grid path

[BenJule]

Problem

When using Fill with copies on small objects (where the grid placement path is used instead of the arrange path), all copies receive the name <object> -1 instead of sequential names like <object> 0, <object> 1, etc.

Root cause: In FillBedJob::process(), the grid path (used when grid_cap > FILL_BED_GRID_PATH_MIN_CAPACITY, i.e. > 50) never updated itemid after placing each copy. The field stays at its initial value of -1 (set in FillBedJob::prepare()), so the setter at line 172:

newObj->name = mo->name + " " + std::to_string(p.itemid);

produces "Cube -1" for every single copy.

The arrange path (used for large objects) correctly assigns sequential IDs via firstfit.hpp → it->get().itemId(item_id++).

Fix

Assign itemid sequentially in the grid path loop, matching the 0-based sequential IDs produced by the arrange path:

m_selected[i].itemid = static_cast<int>(placed);

Added inside the if (cell_idx < empty_cells.size()) block, before ++placed.

Related

Closes #10492