update_current_stock() in delivery_note.py used to call
frappe.db.get_value("Bin", ...) separately for every row in items and
every row in packed_items - so a delivery note with 200 items and 200
packed items made 400 separate database calls on every save.
now it groups item codes by warehouse and fetches bin data with one
query per distinct warehouse, then assigns actual_qty/projected_qty to
each row from that result - same values as before, far fewer database
calls, and no cross-product over-fetch across warehouses.
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