Remove the one-way ore API superseded by the transfer menu

The loose end from #24. withdraw_to_fleet, unload_into_building and
deliver_cargo were the old one-way moves behind "Load ore" and "Store
ore here"; every one of them is a special case of transfer_to_building /
transfer_to_fleet, and none had a caller left outside tests. The
cargo_delivered signal had no listeners at all.

Not just tidying: unload_into_building routed through
deposit_to_building, which does NOT special-case the mothership - it
writes b.stock, while building_holdings() returns `rocks` for a
mothership and ignores stock entirely. So unload_into_building(
mothership_id) would have parked ore in a field nothing reads, with no
error. Nothing called it that way, but it is a plausibly-named function
sitting there to be reached for later. transfer_to_building handles it.

CARGO_LOAD_BATCH stays - the transfer menu's batch buttons use it; only
its role as withdraw_to_fleet's default argument is gone.

The #24 note expected folding these in to be costly for
test_cargo_readout, which places no buildings. It is not: the transfer
primitives work off GameState records rather than spawned nodes, so a
single add_building() in setup is enough and it stays a display test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/_transfer_menu_preview.gd.uid

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